Kant's reflections in Universal Natural History and Theory of the Heavens

The trail of Kant's occupation with the subject leads us first of all back to his Universal Natural History and Theory of the Heavens. Until the 1760s, Kant's works were dominated by general problems relating to the natural sciences (Krafft Reference Krafft and Krafft1971, pp. 179, 179 f.), and this early work, published in 1755Footnote ³, is also dedicated to this subject area. It is astonishing how many later astronomical insights Kant anticipated in this workFootnote ⁴, given that the empirical basis at the time of writing was extremely weak (Krafft Reference Krafft and Krafft1971, pp. 180 f.)Footnote ⁵. It should not be forgotten that the specific subject to which Kant devotes himself, ‘scientific’ cosmogony, was a relative noveltyFootnote ⁶. In this Kant's expositions also differ from the purely cosmological observations of Wright (Reference Wright1971), by which Kant was inspired, but which he knew only from a reviewFootnote ⁷. With the design of a cosmogony the term ‘historia naturalis’ was redefined by Kant, and it was he who ‘first conceived of the entirety of world events as historical’ (Krafft Reference Krafft and Krafft1971, pp. 180 f.).

Kant also differs from the other great thinkers of the modern age, who were scientists as much as they were philosophers, in that he ‘cannot be counted among the great mathematicians’ (Krafft Reference Krafft and Krafft1971, pp. 179 f.), and may not even have fully mastered the Newtonian physics of his timeFootnote ⁸. In those days, this was apparently not yet significant in ‘astronomy’; the process of analogy (drawing conclusions from the known to the unknown) had to fill many gaps (Krafft Reference Krafft and Krafft1971, pp. 181 f.). Kant's theory of the heavens is a type of ‘qualitative’ physics – certainly also applying some quantitative calculations – a surprisingly large proportion of which proved well-foundedFootnote ⁹, and later even became ‘a direct starting-point for modern cosmogony’ (Hamel Reference Hamel2005, 204 f.). Thirty years after the initial publication, it was the astronomer William Herschel who formed views about milky ways and nebulae, which broadly overlapped with Kant's identification of the latter with clusters of stars (Kurth Reference Kurth1956, pp. 58 f.). Dirk Evers wrote: ‘Independently of Kant, forty years after the latter's General Natural History, Laplace formulated a similar hypothesis about the formation of the solar system from a rotating nebula. His theory made a bigger impact than Kant's early work, which was then, however, re-discovered and became known to a wider public. The combination of both drafts to form the Kant-Laplace theory then dominated the cosmology of the 19th century.’ (Evers Reference Evers2000, pp. 75 ff.)Footnote ¹⁰

At this time, before Laplace's famous pronouncement that he no longer needed the hypothesis of GodFootnote ¹¹, theology and astronomy were often intertwined. Newton had averted the possible atheistic consequences of his laws of gravity to the extent that, in his letters to Bentley, he postulated a divine hand, which held the planets on course against the gravitational pull of the sun despite frictional loss (Dick Reference Dick1982, pp. 145 f.)Footnote ¹². And Thomas Wright assumed that the Milky Way had a divine centre around which the Solar systems moved (Wright Reference Wright1971, pp. 168 ff.; Hamel Reference Hamel2005, pp. 210 ff.). Kant, however, dispenses in his reflections with this divine assistance still claimed by WrightFootnote ¹³, and for this very reason he devotes his preface to the question of the role played by God in nature. ‘I did not set out upon this enterprise until I saw myself secure in relation to the duties of religion’ (Kant Reference Kant and Watkins2012, p. 194, 1:222).

His basically deistic solution is as simple as it is brilliant. In a similar vein to Leibniz’ criticism of Newton, he avoids interference by God with nature by praising God as the giver and preserver of the order of the very laws of nature. ‘A God exists precisely because nature cannot behave in any way other than in a regular and orderly manner, even in chaos’ (Kant Reference Kant and Watkins2012, p. 199, 1:228)Footnote ¹⁴. This is also more important to him than ‘bringing the miracles of revelation into the same system as the ordinary laws of nature’ (Kant Reference Kant and Watkins2012, p. 258, 1:304), which he nevertheless attempts selectively in his hypothesis of an early ring of water around the earth by way of explanation of the waters in the heavens, described in the creation story of Genesis 1:6 f. (Kant Reference Kant and Watkins2012, pp. 258 f., 1:303–4).

Kant arms himself against the accusation of intellectual audacity – in seeking to describe the genesis of the cosmos – by referring to the great simplicity of the endeavour compared with the complicated nature of the formation ‘of a single plant or caterpillar’ (Kant Reference Kant and Watkins2012, p. 201, 1:230). His motto is: ‘Give me matter and I will build a world out of it.’ (Kant Reference Kant and Watkins2012, p. 200, 1: 230). With these words, Voltaire criticizes Descartes’ physicsFootnote ¹⁵; Kant's adoption of the quotation shows how he attempts, in all modesty, to honour the intention which Voltaire presents as presumptuous.

In order to do this, the first thing Kant needs are the Newtonian laws, which is why he sets them out again in a brief synopsis before beginning the first part of his work (Kant Reference Kant and Watkins2012, pp. 12–14, 1:243–246). The central idea, which Kant then discusses in Part I itself, is that of Wright. ‘He regarded the fixed stars not as a disorderly mass distributed without any intent, but rather found a systematic constitution in the whole’ (Kant Reference Kant and Watkins2012, pp. 215 f., 1:248), in that he regarded the fixed stars as suns with their own Solar systems (Kant Reference Kant and Watkins2012, pp. 201 f., 1:231), which circulated around a common central point. In the tradition of natural theology and in particular in connection with the poet Alexander PopeFootnote ¹⁶, whom he regarded highly, preceding each of the three parts of his work with a quotation from his poemsFootnote ¹⁷, Kant sees the demonstrated unfathomability of the cosmos as a reason to praise the Creator (Kant Reference Kant and Watkins2015, p. 222, 1:256), who reveals himself ‘in such inconceivably great works’ (Kant Reference Kant and Watkins2012, p. 215, 1:248).

The second part of the General Natural History and Theory of the Heavens then consists of eight chapters, the last of which Kant highlights in the preface as particularly relevant for the sceptic (Kant Reference Kant and Watkins2012, p. 204, 1:234). Here Kant again concerns himself with Newton's divine hand, placing himself within the Natural Theology of his time as follows: ‘One conclusion is quite correct: If order and beauty shine forth in the constitution of the world, then there is a God. However, the other is grounded no less: If this order was able to flow from universal laws of nature, then all of nature is necessarily an effect of the highest wisdom’ (Kant Reference Kant and Watkins2012, p. 292, 1:346). The theology in the work should not surprise anyone, as the whole thing has the layout of a physico-theological proof of the existence of GodFootnote ¹⁸.

Kant's treatise also has a third partFootnote ¹⁹ or appendixFootnote ²⁰, which deals explicitly with the more specific subject of this paper, that is, the possible significance of extra-terrestrial life in the cosmos. Kant himself indicates the status of this part in the preface as follows: ‘Nonetheless, the reader will find somewhat more than mere arbitrariness but somewhat less than undoubtedness in it’ (Kant Reference Kant and Watkins2012, p. 205, 1:236). The subject is later found again in exactly the same intermediate position in the Critique of Pure Reason Footnote ²¹. In the third part, then, like many great thinkers before him (Dick Reference Dick1982), Kant occupies himself with the ‘inhabitants of the planets’ (Kant Reference Kant and Watkins2012, pp. 295 f., 1:352), by developing the principle of conclusion by analogy to such an extent that he not only assumes that most of the planets are inhabited, but also postulates a principle that explains the differences between the inhabitants of the various planets. What Kant developed for the known Solar system is here also applied paradigmatically to the other postulated star systems. Although Kant is aware that ‘the freedom to invent has no real barriers’ (Kant Reference Kant and Watkins2012, p. 295, 1:351) with regard to this subject, and that a person's imagination can certainly run wild in this context, he is nonetheless convinced that the probability is well-founded (Kant Reference Kant and Watkins2012, p. 195, 1:352). In this way, he postulates that the distance between the various planets and the sun must have a considerable influence on their inhabitants, a conclusion he draws from his theory about the formation of the Solar Systems, which also constitutes a reversal of Wright's assumptionsFootnote ²². According to Kant, ‘the perfection of the spiritual world as well as of the material world increases and progresses in the planets from Mercury on to Saturn or perhaps even beyond it (insofar as there are yet other planets) in a correct sequence of degrees in proportion to their distances from the Sun’ (Kant Reference Kant and Watkins2012, p. 302, 1:361). Since the effect of the sun as the ‘source of light and life’ (Kant Reference Kant and Watkins2012, p. 303, 1:362) wanes in proportion to the increasing distance, so, as he sees it, the matter and the composition of the planets and their inhabitants must have a correspondingly superior nature in order to continue to benefit from this effect. This accepts as a given the idea of natural theology that God ‘has so beneficially ordered everything for the good of the rational beings that inhabit [the planets]’ (Kant Reference Kant and Watkins2012, p. 304).

This also means, although Kant is aware of the speculative nature of this conclusion, that sin can only be a problem associated with an intermediate distance from the sun, as the beings inhabiting higher planets would presumably be above and beyond its influence due to their superior materialityFootnote ²³, whereas those inhabiting lower planets would be equipped ‘with far too few spiritual abilities to be permitted to bear the responsibility for their actions before the judgement seat of justice’ (Kant Reference Kant and Watkins2012, p. 306, 1:363). With these concluding reflections Kant also circumvents soteriological problems. It was for such reasons that Philipp Melanchthon, for example, had rejected the assumption that numerous worlds exist (Dick Reference Dick1982, pp. 89 f.).

Kant, too, was of the opinion ‘that it is not necessary to assert that all planets must be inhabited’ (Kant Reference Kant and Watkins2012, p. 295, 1:352)Footnote ²⁴, but he also followed the tradition that the purpose of nature is to be observed by reasoning beings, and therefore it seemed more appropriate to assume that the planets were (to a great extent) inhabited (Kant Reference Kant and Watkins2012, pp. 295 f., 1:352)Footnote ²⁵. In this regard it is possible, according to Kant, that the planets that are uninhabited have merely not yet fully developed, or have already passed a certain stageFootnote ²⁶.

Kant's attitude to aliens in his critical period

The General Natural History makes it clear that the later critiques were written by someone who had himself experienced the temptations of speculative systems (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 19 f.). Kant is at this point increasingly critical of the more speculative observations of his own work. The extract from it published by J. F. Gensichen in 1781 as an appendix to the German translation of essays by William Herschel omits much of the work, not only because of space constraints. ‘The rest, [Kant] says, contains too much in the way of mere hypotheses, for him to be able still to approve it completely.’ (Gensichen Reference Gensichen1791, pp. 201 f.) Parts that were deleted include the preface and several chapters in Part II, including the eighth, and thereby the complete theological framework of the whole work. The whole of Part III was also omitted (Hamel Reference Hamel2005, pp. 167 f.). Anything ‘merely hypothetical’ did not survive. One might have thought that this would also settle the question of the aliens. But the subject of extra-terrestrial life was a lasting one for Kant.

The section of the Critique of Pure Reason containing the quotation cited at the beginning of this paper is entitled ‘On having an opinion, knowing, and believing’ (Kant Reference Kant, Guyer and Wood1998, pp. 684 ff., B 848 ff.)Footnote ²⁷. Here Kant sets out his well-known categorization of believing as something in between having an opinion and knowing, as a merely subjectively sufficient, but objectively insufficient taking to be true (Kant Reference Kant, Guyer and Wood1998, p. 686, B 850). ‘Only in a practical relation, however, can taking something that is theoretically insufficient to be true be called believing’ (Kant Reference Kant, Guyer and Wood1998, p. 686, B 851ff.). Within this believing he makes a distinction between accidental and hypothetically necessary believing (‘if I know with certainty that no one else can know of any other conditions that lead to the proposed end’ (Kant Reference Kant, Guyer and Wood1998, p. 687, B 852)), where what he calls accidental believing is also referred to as pragmatic, because, for example, a doctor occasionally has to act pragmatically without knowing any better.

As a touchstone as to ‘whether what someone asserts is mere persuasion or at least subjective conviction, i.e. firm belief’ (Kant Reference Kant, Guyer and Wood1998, p. 687, B 852) he names betting. This would reveal the degree of personal conviction of one's own assumptions, in particular if all one's possessions were at stake. ‘If we entertain the thought that we should wager the happiness of our whole life on something, our triumphant judgment would quickly disappear, we would become timid and we would suddenly discover that our belief does not extend so far’ (Kant Reference Kant, Guyer and Wood1998, p. 687, B 853).

Analogous to belief in a practical relation stands doctrinal belief, which, according to Kant, refers to purely theoretical judgements and certainly demands a means ‘for arriving at certainty about the matter’ (Kant Reference Kant, Guyer and Wood1998, p. 687, B 853). At this point Kant gives an example of such doctrinal belief in the quotation already cited at the beginning of this paper: ‘If it were possible to settle by any sort of experience whether there are inhabitants of at least some of the planets that we see, I might well bet everything that I have on it. Hence I say that it is not merely an opinion but a strong belief (on the correctness of which I would wager many advantages in life) that there are also inhabitants of other worlds.’ (Kant Reference Kant, Guyer and Wood1998, p. 687, B 853).

The conditional form is to be understood as being necessary here, since it is about a purely theoretical judgement: in the Critique of Pure Reason, Kant does not yet assume that the question of the existence of aliens could ever be settled. Even so, this belief in aliens represents for Kant nothing less than a textbook example of doctrinal belief. In the same way, Kant uses inhabitants of the moon to illustrate transcendental idealism (Kant Reference Kant, Guyer and Wood1998, 512, B 521). This may be surprising from a modern point of view, and for this reason the historical section of this paper will present the context in which Kant was able to come to this view.

In view of this strong belief in the existence of aliens, it is not surprising that, from an epistemological viewpoint, in the case of Kant this belief is initially on the same level as belief in God. For Kant continues unabashed that we must ‘concede that the thesis of the existence of God’ also ‘belongs to doctrinal belief’ (Kant Reference Kant, Guyer and Wood1998, p. 688, B 854). By this Kant means the physico-theological conviction of ‘the presupposition of a wise author of the world’ (Kant Reference Kant, Guyer and Wood1998, p. 688, B 854) when investigating nature, that is, assuming a uniform and meaningful world structure, while constantly striving to ‘make use of my reason as if everything were mere nature’ (Kant Reference Kant, Guyer and Wood1998, p. 688, B 854). Here the positive outcome of natural history experiments serves, he writes, to confirm the viability of this presupposition of a wise author of the world. For this reason, it was too little to call taking something to be true merely ‘having an opinion’, when he, after all, ‘firmly believe[d] in God’ (Kant Reference Kant, Guyer and Wood1998, p. 688, B 854), but this belief should not be called practical but doctrinal – as, incidentally, should belief in a future life of the human soul (Kant Reference Kant, Guyer and Wood1998, p. 688, B 854). And a merely doctrinal belief of this kind ‘has something unstable about’ it (Kant Reference Kant, Guyer and Wood1998, p. 688, B 855), as Kant admits.

But moral belief, he says, is a different matter, as it is absolutely necessary, because otherwise the very moral principles would be overturned; and because belief both in the existence of God and in a future life also belongs to this belief, ‘nothing can make these beliefs unstable’ (Kant Reference Kant, Guyer and Wood1998, p. 689, B 856).

Here there is therefore already a distinction between the category of belief in the existence of aliens and that of the broader belief in God (incorporating a moral dimension) and a future world, which perhaps for this reason in the later Critique of Judgement leads to the assumption that there are rational inhabitants of other planets being declared a ‘matter of opinion’ (Kant Reference Kant and Guyer2000, p. 332, 5:467). While in the Critique of Pure Reason holding an opinion was defined purely in terms of a deficiency as subjectively and objectively insufficient taking to be true (Kant Reference Kant, Guyer and Wood1998, p. 686, B 850), Kant emphasizes in the Critique of Judgement the positive difference between holding an opinion and believing, stating that matters of opinion are ‘objects of an at least intrinsically possible experiential cognition’ – although this cognition is, of course, ‘merely because of the degree of capacity that we possess … impossible for us’ (Kant Reference Kant and Guyer2000, p. 331, 5:467)Footnote ²⁸. The question of the existence of aliens is now no longer a question of belief for Kant, because he now considers this experiential cognition to be verifiable and/or falsifiable. It could therefore even be an object of knowledge: ‘…for if we could approach more closely to other planets, which is intrinsically possible, we could determine by means of experience whether they exist or not; but we never will come close enough to other planets, so this remains a matter of opinion’ (Kant Reference Kant and Guyer2000, p. 332, 5:467)Footnote ²⁹.

But what would Kant have decided if he had been familiar with the facts as they are known today? The discovery of extra-terrestrial life is moving in our day into the realm of the possible. In Kant's terms, it is still a matter of opinion. In any case, it is no longer a matter of belief, but has now become a question that is potentially accessible to science.

Old and new astronomical insights

Why does Kant assume that aliens exist? Today the debate about aliens appears exotic, perhaps because in the 20th century, apart from the dubious comments of believers in UFOsFootnote ³⁰ and the unsuccessful experiments of the SETI project (Wilkinson Reference Wilkinson2013, pp. 83 ff.), there was an unusual silence on this subject. There was also no longer any certainty whether, apart from the stars, there were even any planets on which life could have existed (Wilkinson Reference Wilkinson2013, pp. 27 ff.). In earlier centuries, it was differentFootnote ³¹. The history of the debate about the possibility of extra-terrestrial life shows that Kant's reflections become important in the context of a rich tradition.

Historically, the Copernican revolution had led to a great openness in addressing this question, as it made the Aristotelian cosmology obsolete. Since the earth thereby lost its special position in the centre of the cosmos, it was easy to conceive the existence of several ‘earths’. It was at the core of the new type of astrophysics not to regard the sun, the moon and the planets that had so far been discovered as perfect spheres under motionless heavens of fixed stars, but to apply to them the rules that applied on Earth. This was an expression of the ‘Copernican principle’ or the ‘principle of mediocrity’, according to which, in a basically uniform Universe, the earth does not hold a special position, and which, in a watered-down form, represents no less than the philosophical basis for modern science (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 7 f.). After applying the principle of mediocrity and observing divine omnipotence, even inhabitants of the stars seemed plausible, but there were equally strong theological reservations about assuming Copernicanism to be true. Giordano Bruno was probably the first to maintain that the stars were bodies similar to our Sun, and also surrounded by planets in the same way (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 8 f.); his fate is well known, although the above assertion was undoubtedly not the main reason for his tragic execution (Dick Reference Dick1982, pp. 69 f.). Philip Melanchthon remained faithful to Aristotelianism against Copernicus, in part because he did not find any references to further universes or inhabitants of other stars in the biblical account of the creation. In addition, serious questions were raised with regard to the Christian doctrine of redemption and incarnation (Dick Reference Dick1982, pp. 88 ff.), which Kant's precritical approach so neatly circumvents, because he assumes that sin is a problem only associated with intermediate distance from the sun.

Possibly for similar reasons – although, incidentally, unlike Kepler – Galileo Galilei was also reluctant to postulate inhabitants of other stars, or at least he was warned for these reasons against doing so (Dick Reference Dick1982, pp. 90 f.)Footnote ³². On the other hand, in his defence of Galilei, Tommaso Campanella also found biblical arguments in favour of Copernicanism. Thus the biblical account of the creation speaks of waters above and under the firmament, and Campanella applied the water above the firmament to the presumed water reserves on the moon. Campanella also suggested that the death of Christ on the cross was perhaps not even necessary for the inhabitants of the stars, since the latter were not descended from Adam and were therefore not subject to original sin. In addition, he pointed out that similar ideas had already been discussed with regard to the inhabitants of remote parts of the earth (Dick Reference Dick1982, pp. 90 ff.).

Galilei himself kept out of the discussion, emphasizing the differences between the earth and the moon despite remaining loyal to Copernicanism (Dick Reference Dick1982, pp. 96 ff.). John Wilkins, however, following the groundwork of Giordano Bruno in Protestant England, was able to support the assertion that a world had been discovered on the moon (Discovery of a World in the Moone, 1638) and to confront readers with the following question: If, according to Copernicus, the earth is merely one of many planets, why should one of the many planets not be a second earth? (Dick Reference Dick1982, pp. 97 ff.)

It was René Descartes who in his Principia Philosophiae in 1644 formulated a vortex theory, which then made popular the assumption that numerous solar systems existed. However, his followers did not initially assume that these vortex systems also contained other planets, in addition to the suns. It is possible that the difficulties relating to ecclesiastical politics confronting Galilei contributed to the fact that this idea and the related question of the habitation of the planets were not pursuedFootnote ³³. This changed only with Bernard le Bovier de Fontenelle, the later secretary of the French Academy of Sciences (Dick Reference Dick1982, pp. 123 ff.), and with the Cosmotheoros (1698), the posthumous late work of the highly regarded Christiaan Huygens (Dick Reference Dick1982, pp. 127 ff.). They explicitly assumed that planets also existed, which orbited the suns in the vortices, and together with these planets, they assumed, came inhabitants.

Just 1 year after the first publication of de Fontenelle on this subject, Isaac Newton's Philosophiae naturalis principia mathematica (1687) turned natural philosophy upside down and at the same time pushed the vortex theory into the scientific sidelines (Dick Reference Dick1982, pp. 142 f.). Newton's law of gravitation left the question of multiple solar systems open again, and Newton did not initially tackle the question of cosmogenesis in his works. It was in his letters to Richard Bentley that Newton formulated the assumption that the genesis of the solar systems with their planetary motions would be inconceivable without the help of a ‘divine hand’, thus turning against the basically atheistic hypothesis of the atomists (Dick Reference Dick1982, pp. 142 ff.).

It should not be forgotten that the existence of other solar systems could not at this time yet be observed, but only postulated. It therefore had to be metaphysical principles, which led to this assumption. It was for teleological reasons that Bentley assumed, all heavenly bodies had been created for the purpose of their respective inhabitants. In this way, the assumption of inhabited solar systems became a part of the entity that we know as natural theology, of taking the evidence of the glory of God from the works of nature (Dick Reference Dick1982, pp. 149 f.).

A famous work in this tradition was William Derham's much-read Astro-Theology (1715), which used each astronomical detail as a proof of divinity. It was also Derham who, on a Newtonian basis, popularized the assumption of multiple solar systems as a third ‘new system’ following the Aristotelian and Copernican approaches. The suns were there in order to bestow light and warmth on their own inhabited planets (Dick Reference Dick1982, pp. 151 f.). An echo of these discussions, in the light of which his position certainly also becomes clearer, is finally to be found in Kant's examination of the subject.

William Herschel, the greatest astronomer of the time, was also a passionate apologist for the habitation of other planets, also of the moon and even the sun (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 20 ff.), which he believed to be merely a particularly large and bright planet (Crowe Reference Crowe1999, pp. 67 f.; Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 33 ff.). At the end of the 18th century, belief in extra-terrestrial inhabitants of planets was so widespread that Thomas Paine, in his book The Age of Reason, was able on this basis to attack the Christian faith: ‘to believe that God created a plurality of worlds, at least as numerous as what we call stars, renders the Christian system of faith at once little and ridiculous, and scatters it in the mind like feathers in the air.’ (Paine Reference Paine1794, pp. 84 f.)

What do the heavens declare?

Around the beginning of the 19th century it was therefore no longer at all certain that ‘the heavens declare the glory of God’ (Psalm 19:1), while the ‘population’ of the heavens was at the same time supported by the scientific consensus. When John Herschel, a highly regarded astronomer and philosopher of science, demanded in his Preliminary Discourse on the Study of Natural Philosophy (1830) strict scientific criteria, in particular for astronomy, because its discoveries stood in open contradiction to superficial observation, his example of this is counter-intuitive from a modern perspective: ‘The planets, which appear only as stars somewhat brighter than the rest, are to (the astronomer) spacious, elaborate, and habitable worlds.’ (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 27 f.) Thus, from his viewpoint, it was the in-depth analysis itself that led to the discovery that some stars were in fact habitable planets. August Comte, too, thought that astronomy was the only science that had left theological and metaphysical developmental stages behind, and the heavens declared the glory only of Hipparchus, Kepler, Newton, etc. At the same time, he maintained that it could be taken for granted that the planets of the Solar System were inhabited (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 27 ff.).

By the mid-19th century the tensions between religion and astronomy had to a great extent subsided. It was widely assumed that belief in extra-terrestrial life was not only compatible with religion, but even supported it (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 37 f.), as envisioned, for example, by Thomas Chalmers, in his Astronomical Discourses (1817).

It was then the aliens themselves whose existence was called into question, and this was done by William Whewell, who published the book Of the Plurality of Worlds: an Essay in 1853, linking its title to the predominant tradition – as embodied by such people as Chalmers – only to reject this tradition in the course of the workFootnote ³⁴. This was based on his evaluation of the existing astronomical and scientific facts, which he used for apologetic purposes to re-establish the special position of the earthFootnote ³⁵. What is interesting about this is not so much the recovering anthropocentrism on religious grounds linked to Psalm 8, but the incorporation of the contemporary state of facts in natural sciences and the sense of the speculative nature of many assumptions, which had until then been regarded as scientific: ‘Men have been so long accustomed to look upon astronomical science as the mother of certainty, that they may confound astronomical discoveries with cosmological conjectures; although these be slightly and illogically connected with those.’ (Whewell Reference Whewell1853, pp. 101 f.) In evaluating the geological discoveries of his time, he was in this way even able to challenge the long-established metaphysical principle of abundance, according to which populated worlds give the creator greater glory than empty outer space, since even on Earth, as was now known, there had been numerous populations which – even before Darwin's Origin of Species of 1859 – were regarded as separate new creations (Whewell Reference Whewell1853, pp. 103 f.). By evaluating the facts, Whewell also recognized, for example, that the level of radiation from the sun made the inner planets of our Solar System very hot and the outer planets very cold, and that the low density of Jupiter suggested that it had no surface at all. He therefore already had a notion of the modern concept of the ‘habitable zone’Footnote ³⁶, the area of the Solar System in which habitable planets could exist. However, he also assumed that the hypothesis that nebulous areas in the starlit sky were universes comparable with our Milky Way was false. ‘And thus we are not, I think, going too far, when we say, that our Solar System, compared with spiral nebulous systems, is a system completed and finished, while they are mere confused, indiscriminate, incoherent masses.’ (Whewell Reference Whewell1853, pp. 136 f.) Although he was wrong about this, even at the end of the 19th century his views still corresponded to the predominant opinion of research (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 44 f.).

Whewell provoked a debate, but belief in numerous habitable worlds was not yet permanently shaken by his ideas (Dick Reference Dick2001, pp. 17 f.). However, the tendency is clear: science was now beginning to regard the existence of numerous habitable worlds as improbable (Wilkinson Reference Wilkinson2013, pp. 27 f.). When it became increasingly clear that the Sun, due to its temperature, and the Moon, due to its lack of an atmosphere, could not host any life, attention was then turned to Mars, on which Giovanni Schiaparelli first claimed to have observed canals in 1877. The public discussion of this issue led amongst other things to H.G. Wells’ classic The War of the Worlds (1897), but at the turn of the 20th century the opinion took shape that the alleged canals were optical illusions (Crowe & Dowd Reference Crowe, Dowd and Vakoch2013, pp. 46 ff.). Thus, the aliens were ‘expelled’ from the Solar System at this time. The number of expected planetary systems in the Universe had also significantly decreased due to the abandonment of the hypothesis that the nebulae represented separate galaxies, and the remaining fixed stars were also not able to host any life due to their Sun-like nature. It was science that made the idea of numerous worlds become improbable. But it was also science that took up this idea again, following new discoveries.

Challenges today

It was not until 1995 that the first planet outside our Solar System, orbiting a star similar to the Sun, so the first real ‘exoplanet’, was discovered by Swiss astronomers Michel Mayor and Didier Queloz. Through the use of space telescopes, in particular, about 2000 exoplanets have now already been detected, and it is believed that approximately one in every ten stars has at least one planet (Batalha Reference Batalha2014, p. 12647). Since the Milky Way contains at least 300 billion stars (Gribbin Reference Gribbin2008, p. 28), that would still add up to at least 30 billion planets existing in our Galaxy alone. The habitability of the planets, that is, whether they lie within the ‘habitable zone’, depends crucially upon their distance from the relevant Sun. In our Solar System, for example, Mars is also situated just within this zone, and is certainly a good example of Kant's assumption that the planets that are uninhabited may already have passed beyond a certain stage. Thus even today it is believed in the case of Mars that it is quite possible that it may once have hosted life (Wilkinson Reference Wilkinson2013, pp. 9 ff.). Even if only a fraction of the existing planets could host life, and of those, again only a fraction actually did, in our Galaxy alone that would still amount to a substantial number of planetsFootnote ³⁷. However, this then raises the question of where all the aliens are. This paradox has led Enrico Fermi, amongst others, to assume that in our Galaxy there is no other intelligent life. However, there are also other approaches to responding to this paradox (Webb Reference Webb2002).

Whatever the case may be regarding life beyond our planet, the question has been treated as a matter for ‘exobiology’. However, since no life other than that originating on Earth has yet been found in space, the purpose of this discipline has been questioned, some maintaining that it seemed to be the only scientific discipline of which it was not possible to be certain that its subject actually existedFootnote ³⁸. The discipline was therefore re-invented under the heading of ‘astrobiology’, in which life on and from earth is considered in the context of outer space (Catling Reference Catling2013, pp. 5 f.). As has been demonstrated, however, outer space is rather large, and the total absence of life elsewhere must be somewhat improbable, especially since life on Earth is based on the most common elements in the Universe (Catling Reference Catling2013, pp. 9 ff.). On the other hand, due to the vast distances between solar systems, it is possible that we may never be able to come into contact with any extra-terrestrial life, at least according to our present-day understanding of future technological possibilities.

Theological study of the implications of the possible existence of extra-terrestrial life is carried out in line with the biological sub-disciplines under the headings of ‘exotheology’ (Cf. Peters Reference Peters1994, p. 1) or ‘astrotheology’ (Peters Reference Peters2014, p. 443). The necessity of such a discipline becomes clear when the popular physicist Paul Davies, for example, comments that ‘The existence of extra-terrestrial intelligences would have a profound impact on religion, shattering completely the traditional perspective of God's special relationship with man. The difficulties are particularly acute for Christianity, which postulates that Jesus Christ was God incarnate whose mission was to provide salvation for man on Earth. The prospect of a host of ‘alien Christs’ systematically visiting every inhabited planet in the physical form of the local creatures has a rather absurd aspect. Yet how otherwise are the aliens to be saved?’ (Davies Reference Davies1983, p. 71) One is reminded of Thomas Paine's objections at the end of the 18th century. The theologian Ted Peters, on the other hand, sees numerous possibilities for approaching the proposed task in a constructive way, and refers to the reflections of numerous theologians, including modern ones, in this direction (Peters Reference Peters1994).

So, what is the challenge for theology? There are basically five different scenarios (with differing epistemological status), each of which presents its own challenge to theology:

Scenario 1: There is no other life in space. This negative scenario is often assumed, but cannot be epistemologically verified, because the Universe is simply too big (Akerma Reference Akerma2002, pp. 285 f.). The assumption of this possibility does, of course, make it possible to preserve the old habits of thought and to believe that the earth and mankind are something quite special in the Universe, but it can also lead to a horror vacui, if one thinks, for example, of Jean Paul's Speech of the Dead Christ from the Universe (Richter Reference Richter1826, p. 154).

Scenario 2: Apart from life on Earth, there is only ‘simple’ life in space. Although it would be possible to verify such life provisionally, in this case one would also never know whether there was more complex life somewhere in the Universe, which would then negate the scenario. All the same, in the event that such ‘simple’ life were to be discovered, it might be possible to settle the question of the origin of life on Earth, and in a certain sense, therefore, that of Genesis. A precondition for this would be that great care be taken to avoid contamination from one side to the other; this is an example of a field of ‘astroethics’ that would need to be developed (Peters Reference Peters, Impey, Spitz and Stoeger2013, pp. 200, 201 ff.).

Scenario 3a: There is intelligent life beyond our planet, but it is too far away to be detected. It is not currently possible to distinguish epistemologically between Scenarios 1 and 3a. It would therefore be wise, in view of the probabilities mentioned above, to assume Scenario 3a. This could, following technological advances, be transformed into Scenario 3b. In view of the discovery and increasing mapping of the exoplanets, these technological advances could perhaps even be achieved within our lifetime.

In Scenarios 1–3a, we are in the area of Kant's ‘having an opinion’. We have seen that it was in the very process of his increasingly critical reflections on the subject that Kant realized that the existence of extra-terrestrial life should not be the subject of pure speculation, and is not a question of belief, but will one day in principle be verifiable, and could therefore be transformed into knowledge. This is covered by the following two scenarios: 3b and 3c.

Scenario 3b: There is intelligent life beyond our planet and we can detect traces of it, but it is too far away for us to make contact with it. This situation could occur due to technological development in one of the affected civilizations, and through this development it could also be transformed one day into Scenario 3c. In this case, mankind ought to prepare itself for the subsequent scenario, but theological reflections would be essential beforehand. This would re-open on an interplanetary scale the questions of religious dialogue on Earth; thus the theology of the religions can already give this matter some important preparatory thought.

Scenario 3c: There is intelligent life beyond our planet, and we can make contact with it. The question then would be, friend or foe? This may also depend on the type of dealings in the first meeting, and in addition to the theological issues, raises the question of the development of a comprehensive, appropriate field of astroethics (Peters Reference Peters, Impey, Spitz and Stoeger2013, Reference Peters2014, pp. 454 ff.). The postulated transitions between the different scenarios do, of course, contain certain optimism with regard to progress, which in my opinion appears justified due to the very rapid technological evolution of mankind on a cosmic scale.

The assumption of extra-terrestrial life currently, however, continues to be an object of speculation, and Kant's wager is still open. In the last two scenarios, Kant would win his wager. For the Christian faith, however, what would be at stake in this case, for example regarding the concept of the incarnation? It would certainly require a separate paper to present thisFootnote ³⁹. Precisely by reviving this old debate, the field of theology can prepare itself for the probable or at least possible future, in order to make a contribution to the orientation of mankind.

However, in conclusion it should still be borne in mind that it is of course a question of principle whether faith should even be interested in these issues. In view of the predominant paradigm in continental Europe of mutual independence between theology and science this question is not in the least trivial. In times of Nazi Germany, it was a most important contribution of Karl Barth's theology to insist on the ‘one word of God’ and reject assimilation to the ‘German-Christian’ spirit of the age, resulting in the Barmen declaration. Would it not be a betrayal of that theological legacy, if such important events for the global community as the discovery of extra-terrestrial life were also given a potential theological significance? In any case, in the development of the theology of religions one would have to be very vigilant about keeping to the word of God alone ‘as a source of Church proclamation’ (Barth Reference Barth1957, p. 175). Karl Barth himself went through this, using examples in his doctrine of lights (Barth Reference Barth1961, Section 69 The Glory of the Mediator). If one wishes to adhere to the revelatory nature of the word of God, one must not give up the universalist claim of biblical belief, otherwise the monotheistic concept of God will also be given up. Thus, even if the inhabitants of alien worlds have their own suns, the revelation and illumination must come to them from the single Source of Light, which also appeared to us here on Earth.