This rich and welcome book is an English translation, by the late Gerald Parks, of a revised version of Maria Rosa Antognazza's Trinità e Incarnazione: Il rapporto tra filosofia e teologia rivelata nel pensiero di Leibniz (Vita e Pensiero: Milan, 1999). It is a historical-philosophical account of Leibniz's writings on the Trinity and incarnation doctrines, including his mostly unpublished comments on the controversial writings of others. The approach is historical rather than topical, which introduces some repetition; those interested in pursuing specific arguments or topics in detail will find themselves flipping around a lot, and frequently diving into the copious endnotes. Those interested in the historical angle will appreciate these endnotes (occupying 112 of the book's 322 pages), the fruits of countless hours chasing down and translating obscure manuscripts. And those who only (or primarily) read English will appreciate her broad scholarship, which draws on recent German, French, and Italian secondary literature. The book sports a solid index, and is clearly written and organized. The main audience will be those interested in historical philosophical theology, particularly readers of Leibniz's ‘Preliminary discourse on the conformity of faith with reason’ which begins his Theodicy. Readers of Dixon's (2003) book, Nice and Hot Disputes will be interested as well, as she also expounds Leibniz's thoughts on the fascinating Trinitarian controversy among Anglicans in the 1690s.

Antognazza reveals a Leibniz who is a confident, but careful and tolerant apologist for traditional Christianity. Not unlike present-day Christian analytic philosopher-apologists, Leibniz never tires of claiming that these doctrines haven't been proven contradictory, taking this to be the main point of unorthodox interlocutors – that they are demonstrably contradictory.

In the face of sophisticated objections, he's quick with the logical judo, in a way which is not always convincing. As an example, Leibniz considers this argument by Polish Socinian Andrew Wissowatius (a.k.a. Andrew Wiszowaty) (1608–1678): ‘The one most high GOD is that Father from whom all things come. The son of GOD JESUS CHRIST is not that Father from whom all things come. Therefore the Son of GOD JESUS CHRIST is not the one most high God’ (22). A natural way (at least, to most present-day philosophers) to analyse this argument is as follows:

1. (1) Fg (Fx means ‘x is that Father from whom all things come’)

2. (2) ~Fc (g names God, and c names Christ)

3. (3) Therefore, g≠c.

If something is true of God that isn't true of Christ (or vice-versa), then it follows (by Leibniz's Law – that is, by the indiscernibility of identicals) that God and Christ are not numerically identical. Alternately, we might read the premises as identity statements:

1. (1) g=f

2. (2) s≠f

3. (3) Therefore, s≠g.

Here (3) follows by the transitivity of identity, a necessary truth on which Leibniz often and rightly insists. Both arguments are valid. But Leibniz doesn't admit either analysis. He urges that Wissowatius's argument should be read like this:

1. (1) Everyone who is the one most high God is that Father from whom all things come.

2. (2) The Son of God Jesus Christ is not that Father from whom all things come.

3. (3) Therefore, the Son of God Jesus Christ is not the one who is the one most high God. (25)

This argument seems valid as well. But Leibniz thinks that this formulation reveals an ambiguity in premise (1), concerning the scope of the universal quantifier (Latin: omnia – all or everything), which enables him to claim the argument is valid but turns out unsound however the ambiguity is resolved. If by omnia we mean only the creatures (and thus, not the Son, who is eternal and uncreated), Leibniz denies (2). (The Son is the source or ‘father of’ all creatures.) But if omnia includes the Son as well, he denies (1). (The Son is the one God but isn't the source off all things including himself; rather, he comes from the Father.) Antognazza observes that ‘Leibniz's ultimate aim seems to be the denial of [premise (1)]’ (26). As he says in a later text,

So the Son is not God the Father (some things are true of each, which are not true of the other), and yet the Son is ‘the same one who’ is the Father. In short, Son and Father can be the same being (even the same ‘who’, the same person?) without being identical. If this is his strategy, Leibniz could simply admit either of my two analyses above as sound, but consistent with the doctrine of the Trinity. But I wonder if Leibniz here isn't simply failing to engage his opponents, who probably assume there's no difference between being the same being, and being numerically identical.

Leibniz considers the Trinity and incarnation doctrines ‘mysteries’, which means that they are (one or more of these): (1) not completely understandable by humans, (2) apparently (but not really) contradictory, (3) claims the meaning of which we have but the smallest grasp, (4) not provable or demonstrable, (5) unexplainable, (6) contrary to common notions, (7) improbable. (It is often unclear precisely what Leibniz means by calling a claim ‘incomprehensible’ or ‘a mystery’.) The Christian theologian needn't be embarrassed by these mysteries, for nearly everything in the natural world is a mystery (i.e. it or its essence isn't completely understandable by humans in this life). Unlike some fans of mystery, Leibniz is sensitive to the point that one cannot believe that P (at least, in the sense in which believers should aspire to believe important revealed truths) unless one at least to some degree understands the meaning of P. His solution is to suggest that humans may have ‘confused knowledge’ (as he sometimes puts it, clear but not distinct knowledge, or an ‘analogical understanding’) of the meaning of the terms occurring in these doctrines. This ought not distress us – many philosophical terms are equally poorly understood (56). At his most conservative, Leibniz seems disinclined to explicate the meaning of ‘divine person’ at all. An explication ‘of the Mysteries of religion is not necessary’, Leibniz says at one point, and ‘the safest thing is to stay with the terms of the scriptures and of the church’ (105).

However, the metaphysician in Leibniz will not be repressed. For one thing, one may seek for ‘images’ of these realities in the human mind (107–110). And in bolder moods Leibniz will sometimes (again, like many recent philosophical theologians) suggest a seemingly consistent rational reconstruction, interpretation, or explication (he and Antognazza often say ‘explanation’) of the doctrine of the Trinity. His favourite such move is the claim that the doctrine posits three ‘relative substances’ (or ‘relative beings’) but only one ‘absolute substance’ (‘absolute being’). Yet he seems to back off from this formulation, saying that only the latter is properly called a substance, and three ‘persons’ are ‘understood through incommunicable relative modes of subsisting’ (79), and are ‘constituted’ by their relations to one another (118). Then there is the undeveloped suggestion that the ‘persons’ of the Trinity are not substances (at all?) but rather ‘active principles’ which in some sense compose the one divine substance (158, 110). This reader has the impression that by the time of his mature ‘Preliminary dissertation’, Leibniz had lost some of his enthusiasm for such ‘explanations’ (i.e. plausible metaphysical accounts of) the Trinity and incarnation doctrines, as there he sticks almost entirely to his mysterian defences.

How does his mysterian defence of the rationality of the Trinity and the incarnation work? Leibniz admits in various places that these doctrines are barely understood, apparently contradictory, contrary to appearances and to ‘common notions’, and (antecedently?) improbable. Despite all this, Leibniz's main strategy, in both his ‘Preliminary dissertation’ and in many fragmentary previous writings, is to urge that these doctrines are reasonably believed unless demonstrated to be contradictory.

If this is the game the apologist is playing, he'll find it relatively easy to win, for (as is now widely agreed) there are few demonstrations (roughly, arguments which no sane and unbiased adult human who understands them can doubt to be valid and sound) in philosophy or theology. For nearly any alleged demonstration, one can find a doubtable premise, thus showing the argument to not be a demonstration, even if the argument is in fact sound and indeed convincing to many.

In any case, the above factors constitute prima facie evidence against the doctrines in question. Leibniz accepts this, but holds this evidence to be outweighed by superior evidence to the contrary. He thinks that atheism needn't worry us, for the existence and perfection of God are demonstrable. Further, there are arguments for the truth of Christianity which, while not demonstrations, can be called ‘proofs’, as they give us ‘moral certainty’ of truth of Christianity. A demonstration that, say, the Trinity was contradictory would outweigh any such ‘proof’, but happily there are no such demonstrations. These undefeated proofs ‘justify, once and for all, the authority of Holy Scripture before the tribunal of reason, so that reason in consequence gives way before it … and sacrifices thereto all its probabilities’ (‘Preliminary dissertation’, s. 29). In short, these arguments are ‘incomparably stronger’ than any the dastardly Socinians (etc.) will ever suggest (s. 37).

His whole mysterian defence, then, rests on apologetic arguments for the inspiration of scripture, something like an argument from indirect testimony (to the ministry-validating miracles of Jesus and others). One fears that Leibniz was a better logician than epistemologist. But even if he's right about the strength of those arguments, does the Bible in fact teach the (traditional, creedal) Trinity and incarnation doctrines? Many of Leibniz's contemporaries had argued in depth about this, notably Stephen Nye in his A Brief History of the Socinians (1687, 1691), but Leibniz rests his case on what Antognazza calls ‘the argument from providence’ – that a good God simply wouldn't let His church go astray on matters as central to human salvation as these (75).

One wonders whether a Protestant like Leibniz can consistently affirm such tight providential oversight of (mainstream or widespread) Christian teaching. But the deeper point is that Antognazza's book reveals a lost opportunity. Leibniz was so firmly entrenched in his traditional apologist's defences that he seems to not have understood the perspective of (usually spatially and/or temporally distant) unitarian opponents. They held the Trinity and incarnation to be underivable from the Bible, and this was not solely because they (usually) held the those doctrines to be contradictory, but rather because of the language and doctrines of the New Testament considered as a whole. The English unitarians in which Leibniz was interested (91–110) repeatedly insist that they're not against mysteries (in any of the above senses) per se, but rather against mysteries which are of merely human origin. Nor did they neglect tradition; they were eager to show their views to be compatible with elements of both patristic and (at least some) modern theology. Leibniz does half-heartedly venture a few conventional exegetical arguments but these would and should not have impressed his opponents (115–116).

A minor complaint about the book is that Antognazza, perhaps sticking too closely to her role in reporting Leibniz's views, sometimes passes on his contentious, misleading, or false claims about various ‘anti-Trinitarians’. For example: the Socinians are revivers of ancient Arianism, who stupidly confuse ‘above reason’ with ‘against reason’ and incomprehensibility with inconsistency, think that impossibility follows from improbability, and cavalierly dismiss as textual corruptions biblical passages which affirm the creedal doctrines.

On the whole, though, Antognazza's sympathy for Leibniz's project helps her to present his case with clarity and thoroughness, revealing him to be one of the greatest early modern apologists and philosophical theologians. When push comes to shove, she and Leibniz do carefully present unitarian inconsistency objections to the Trinity and incarnation based on considerations about identity, omniscience, aseity, and so on. Those interested in either metaphysical or mysterian defences of these doctrines would do well to read this unique and well-crafted study.