The main focus of Kant’s Perpetual Peace is on the various political conditions that must be satisfied for a state to be in a condition of perpetual peace. The six ‘preliminary articles’ contained in its first section address specific political issues such as (1) the conditions under which peace treaties can be made, (2) the means by which one state can and cannot acquire another, (3) whether standing armies should be abolished, (4) whether national debt should be allowed to finance war, (5) whether one state can interfere in the constitution and government of another and (6) what kinds of acts of hostility are and are not permitted during war. The three main ‘definitive articles’ then specify that (1) the civil constitution of every state must be republican (rather than democratic), (2) the right of nations must be based on a federalism of free states (rather than a world state) and (3) cosmopolitan right should be limited to universal hospitality (rather than extended more broadly). These main sections are followed by two supplements and a two-part appendix. The first supplement discusses the mechanisms that lead to perpetual peace, while the second recommends (perhaps self-servingly) that political leaders secretly seek the advice of philosophers by allowing them to speak freely in public about how the state should wage war and make peace. The appendices conclude with a discussion of the relations between political and moral considerations, arguing that one should be a moral politician (by ensuring that the principles of political prudence are consistent with moral principles) rather than a political moralist (by adjusting one’s moral principles to suit the politician’s advantage).

Wolfgang Ertl’s excellent book focuses on the first supplement, providing a close textual reading of several particularly dense passages, including one lengthy footnote that could, at first blush, seem like a superfluous digression into esoteric topics. Though the focus of Ertl’s book might thus seem to be narrow, it actually discusses an unusually broad range of topics, including foundational metaphysical, epistemological and theological issues which Kant was thinking about throughout his entire career. It thus makes sense for Kant (and Ertl) to relate these more political matters to his basic metaphysical views. What is more, Ertl’s treatment of these topics turns out to be crucial to understanding Kant’s philosophy as a whole, as it offers, among much else, a reconstruction of Kant’s argument for the perhaps surprising claim that, despite our radical freedom, we should think that nature guarantees perpetual peace at some point in the future.

Ertl’s discussion is structured around three central questions. First, what elements, both natural and otherwise, does Kant foresee playing a role in establishing perpetual peace? Second, if we are to take nature, guided by providence, to guarantee perpetual peace, how could that be consistent with the distinctive kind of libertarian (or contra-causal) freedom that Kant accepts? Third, what is Kant’s argument for this guarantee of perpetual peace?

Ertl’s answer to the first question starts from Kant’s claim: ‘In the mechanism of nature to which the human being also belongs (as a sense entity) a form manifests itself which … we cannot render comprehensible other than by putting the end of a world-author underneath it who determines it beforehand’ (AA, 8: 361).Footnote ¹ Ertl suggests, plausibly, that the ‘form’ that is manifest in nature is constituted by specific laws of nature, that we can comprehend these laws only if we view them as determined in advance by God, that is, by a providential act, and that God does so for the sake of an ultimate end, which Ertl identifies with the free acquisition of a good will by (at least some, if not all) human agents. Drawing on medieval and early modern philosophical theology, Ertl clarifies Kant’s understanding of providence by distinguishing between concurrentist and conservationist positions. Concurrentists maintain that God and natural things cooperate in producing a given effect such that neither on its own is sufficient, whereas conservationists maintain that God creates and conserves finite things in nature that are then the sufficient causes of their effects, without God contributing directly to the effect. Ertl then explains how, in a theoretical respect, Kant rejects concurrentism (whether in a Thomist or Molinist guise) and accepts conservationism, since natural objects can be sufficient for their effects without God’s cooperation, and how, in a moral-practical respect, he accepts concurrentism, since divine grace requires not only that God create and conserve us, but also that God make up for our moral deficiencies. Thus, on Ertl’s interpretation, nature has a form that we can render comprehensible but only if, and because, God conserves the specific laws that are somehow in the service of God’s ultimate end.

But if God and nature act so as to bring about such an end, how can human beings still be free in pursuing it (or not)? To answer this question, Ertl helpfully distinguishes five different ways in which our free agency could be threatened on this picture, three stemming from God’s actions, two from causes other than God. First, God’s foreknowledge of what actions human beings will perform might seem to preclude the freedom of their actions, for if God knows that I will do A and knowledge implies truth, then it might seem that I have to do A. Second, if God determines my existence in creating me, then it might seem that God determines how I act, which can seem to preclude the possibility that I act freely. Third, if God not only creates me but intervenes in the world, then those interventions could preclude my free actions. Fourth, natural causes could prevent my free actions, either because natural determinism is true or because natural causes (whether internal or external to me) necessitate (and thus coerce) my actions. Fifth, the truth-aptness of propositions about future human actions might be incompatible with our freedom: if it is true that I will do A tomorrow, the truth of this proposition might make it impossible for me to do otherwise. Ertl addresses all five threats, noting that Kant devotes most of his attention to the fourth.Footnote ²

Ertl carefully shows how metaphysical issues concerning causation (at both the phenomenal and noumenal levels) and theological issues concerning divine action are central to Kant’s complex answers. Though the details are intricate and deserve further attention, the overall picture is as follows. One crucial part is that the special laws that are supposed to bring about the ultimate end of creation are not independent of our free acts. For on Kant’s view, the empirical laws of nature depend on the natures of things, including our natures, which we freely choose, so that in this roundabout way, the specific laws of nature do depend on us, counterintuitive though that idea may seem at first sight. As a result, rather than the specific laws of nature determining our actions, our free actions determine (in part) the specific laws of nature, for if our free actions were different, then the laws of nature would be different too.Footnote ³ This familiar (though not uncontroversial) part of Kant’s picture helps neutralize the fourth threat in particular.

A second crucial part of Kant’s picture, according to Ertl, concerns the nature of God’s providence. God’s foreknowledge includes God’s knowledge of the intelligible character of free agents, that is, knowledge of what free agents would do under different circumstances, but without God’s knowledge thereby causing the free agent’s intelligible character, which the agent alone is responsible for (setting aside the effects of God’s grace). God’s act of creation is nonetheless relevant to the specific laws of nature because different intelligible characters are possible and, along with them, different specific laws; accordingly, it is up to God to discern which intelligible characters and thus which specific laws to make actual.

Putting these two parts together, we can see that on Ertl’s picture of Kant, because God knows what free agents will do and what specific laws of nature will emerge from the intelligible characters God has created, God knows perfectly well how creation unfolds – God need not wait in suspense to see how it will all turn out. But because of the radical independence of the intelligible character of human beings that derives from our libertarian (or contra-causal) freedom, God is not the cause of the free actions of human beings; despite the fact that God creates human beings, God depends on their free cooperation for the actualization of his plans. In short, God’s providence, taken in both an epistemic and a metaphysical sense, is consistent with human freedom, thereby neutralizing the first, second and third threats to human freedom. Ertl thus presents a clear and compelling case for thinking that freedom is consistent with divine providence.

But what reason can Kant have for asserting that we must take nature, created and sustained by God in this way, to inevitably lead to perpetual peace, a political condition? Given the picture that Ertl has painted, Kant can be viewed as arguing that, in light of God’s omniscience, we can be sure that God has created those who freely choose intelligible characters that will lead to the actualization of the ultimate end of creation, the free acquisition of good wills. Ertl connects this line of thought to the guarantee of perpetual peace by arguing that the ultimate end of creation can take place only in what Kant calls an ethical commonwealth (‘God’s kingdom on earth’, cf. AA, 6: 93), and insofar as a stable good will cannot be achieved by free agents in isolation, perpetual peace is a necessary condition of the ethical commonwealth. In short, because the ultimate end that God has in mind for creation requires that our embodied selves coexist in an institutional legal setting that sustains perpetual peace, God selects those empirical laws that make such getting along inevitable.

This outline of Ertl’s sophisticated interpretation of Kant leaves out much that is of great interest. For example, Ertl clarifies that many of the claims that Kant is committed to in his account of the guarantee of perpetual peace do not amount to cognition, but are rather part of Kant’s practically grounded metaphysics, a reflection of how we must think of things (in the context of thinking about how reason might develop over time in the species), without being able to cognize that this is how they in fact are. However, this brief summary of Ertl’s book should still make clear that, even in a relatively short treatise devoted to specific political conditions, Kant is deeply concerned with showing how the goal that these conditions promote, namely perpetual peace, is made possible only by a much broader range of fundamental issues concerning God (especially his providence), nature (especially its particular laws) and freedom (especially its radical independence from God and its consequences for the laws of nature). Our understanding of Perpetual Peace would be seriously incomplete without a full appreciation of their import and Ertl’s philosophically rich book deserves credit for contributing to our understanding of Kant’s thoughts on this topic.

Ertl’s insightful description of the larger picture against which Perpetual Peace should be read makes it tempting to raise questions about some of its details, given that any good book raises as many questions as it answers. First, in an infrequently discussed passage in the Second Analogy Kant claims that creation ‘cannot be admitted as an occurrence among the appearances, for its possibility alone would already undermine the unity of experience’ (A206/B251), which raises the question of whether Kant can endorse a conservationist position with respect to phenomenal substances, as Ertl maintains. Might Kant not be a conservationist only about noumenal substances? This option is especially attractive to those who, unlike myself, deny the existence of phenomenal substances. Second, it is plausible, in my view, to take God as a partial ground of the specific laws of nature, because such a position is necessary to explain the possibility of the Highest Good (specifically for proportioning happiness to virtue). But how does God’s grounding of specific laws of nature for the sake of the Highest Good relate to God’s grounding of specific laws of nature for the sake of perpetual peace? Are the two grounding relations entirely distinct? Is one more fundamental than the other? These questions become (even) more complicated if one recalls Kant’s claim that it is our unsocial sociability that is responsible for the development of reason, for then one wants to understand how our unsocial sociability relates to the specific empirical laws that go into our empirical natures. Finally, one might concede that, if the highest end of creation requires our embodiment, then perpetual peace would be necessary for this highest end, but why should embodiment be necessary?Footnote ⁴ Though it would be interesting to have full answers to these questions, it is certainly possible that in the end these questions are addressed less to Ertl than they are to Kant.