If the eighteenth century is to be seen as the “Age of Reason,” then one of the crucial stories to be told is of the trajectory of philosophy from one of the most ardent proponents of the powers of human reason, Gottfried Wilhelm Leibniz (1646–1716), to the philosopher who subjected the claims of reason to their most serious critique, Immanuel Kant (1724–1804). Not only is the story of Kant’s Auseinandersetzung with Leibniz important historically, it is also important philosophically, for it has implications about the nature and possibility of metaphysics, that branch of philosophy concerned with fundamental questions such as what there is, why there is anything at all, how existing things are causally connected, and how the mind latches onto the world. Like many philosophical debates, however, it is also prone to a kind of “eternal recurrence” to those who are ignorant of it.
Leibniz was a “rationalist” philosopher; that is, he was committed to two theses: (i) he believed that the mind has certain innate ideas—it is not, as John Locke and his fellow empiricists say, a tabula rasa or blank slate; and (ii) he believed in—and, in fact, made explicit—the “principle of sufficient reason,” according to which “there is nothing for which there is not a reason why it is so and not otherwise.” This principle had enormous metaphysical consequences for Leibniz, for it allowed him to argue that the world, as a series of contingent things, could not have the reason for its existence within it; rather there must be an extramundane reason—God. Further, as a response to the mind-body problem, Leibniz advanced the theory of “pre-established harmony,” according to which there is no interaction at all between substances; the mind proceeds and “unfolds” according to its own laws, and the body moves according to its own laws, but they do so in perfect harmony, as is fitting for something designed and created by God. Strictly speaking, however, Leibniz was not a dualist; he did not believe that there were minds and bodies—at least not in the same sense and at the most fundamental level of reality. Rather, in his mature metaphysical view, there are only simple substances, or monads, mind-like beings endowed with forces that ground all phenomena. Finally, according to Leibniz, since these simple substances are ontologically primary and ground the phenomena of matter and motion, space and time are merely the ordered relations derivative of the corporeal phenomena. Leibniz contrasted his view with that of Isaac Newton, according to whom there is a sense in which space and time can be considered absolute and space can be considered something substantial.
In his Critique of Pure Reason (1781; 2nd ed. 1787), Kant presented a revolutionary philosophical view, one that challenged rationalist and empiricist orthodoxy and one that, he believed, provided answers to certain questions that had been the subject of perennial conflict. Kant advocated “transcendental idealism,” according to which space and time are forms of sensibility—that is, everything that is given to our minds is given as an object in space and time—and there are certain (innate) pure concepts of the understanding that make experience possible. For example, Kant agreed with the empiricist David Hume, who had argued that the idea of cause or necessary connection cannot be traced to an immediate sense impression; yet he believed that the concept of cause must be in our minds in order for us to experience the world at all. A proposition expressing universality and necessity (as in claims of causation) is objectively valid not because of what we experience out there but because of the way our minds function. Kant also argued that there is an important distinction to be drawn between the appearances, or phenomena, and things in themselves, or noumena. According to Kant, we can only have knowledge (Erkenntnis) of things in the phenomenal realm or of the conditions for the possibility of experience; things outside of space and time or beyond the bounds of sense—the “supersensible”—are off limits for our knowledge claims. Thus, in Kant’s view, metaphysics as a science of the supersensible as Leibniz and others had conceived it is doomed to failure; but a metaphysics that first establishes the boundaries of our knowledge claims and determines the conditions for the possibility of experience may indeed succeed.
The importance of the Critique of Pure Reason cannot be disputed. This work fundamentally changed the way philosophers think of the big-ticket items of metaphysics—the existence of God, the immortality of the soul, the freedom of the will. Leibniz believed that we can know that God exists, that the soul is immortal, and that there is freedom (though in a mitigated sense, and here Leibniz is aware that certain aspects of his system lead him into trouble on this topic). Kant, on the other hand, believed that we have theoretical knowledge of none of these things, and in the Critique of Pure Reason famously showed how all arguments for the existence of God are flawed, that the received views about the simplicity and immortality of the soul were problematic, and that our freedom could never be cognized. Yet, all is not lost, for Kant believed that the existence of God, the immortality of the soul, and the freedom of the will must be considered as postulates of morality and not objects of knowledge. In this sense, for Kant, there is a primacy of practical philosophy (ethics) over theoretical philosophy.
While all previous philosophers were, in Kant’s mind, guilty of various errors, Leibniz occupied a special position in his conception of the history of philosophy and the history of reason’s pretensions. Indeed, according to Kant, Leibniz’s metaphysical system was “an intrinsically correct platonic concept of the world.” What exactly was the problem? In a short chapter of the Critique of Pure Reason, “On the Amphiboly of Concepts of Reflection,” Kant makes explicit this disagreement. Put briefly, Kant believed that Leibniz failed to undertake a real critique of the powers of the human mind, failed to see the fundamental distinction between sensibility and understanding, considered perceptual experience a kind of conceptual confusion, and ultimately mistook appearances for things in themselves. More exactly, Kant argued that Leibniz’s metaphysical system is the consequence of the fallacy of “amphiboly”—a fallacy that arises from ambiguities in grammatical form. Consider the following Marxist claim: “Last night I caught a prowler in my pajamas. What he was doing in my pajamas, I don’t know.” The joke, of course, turns on the ambiguity in the phrase “in my pajamas”—does it modify the speaker or the prowler? In a somewhat similar way, Kant argued that because Leibniz failed to distinguish the faculties of sensibility and understanding and to recognize the unique contribution of each in judgment, he attributed concepts appropriate to objects of the understanding to objects of sensibility and thereby made fallacious arguments. For example, Kant saw an amphibolous use of the concepts of comparison of identity and difference, which he argued allowed Leibniz to falsely extend beyond its legitimate scope the principle of the identity of indiscernibles (if two things are qualitatively identical, that is, indiscernible, then they are numerically identical). According to Kant, Leibniz, comparing the concepts of two individuals with each other, argued that they must be distinct, that is, discernible; and he then extended this principle to objects of the senses and claimed to have made a great discovery about nature: “there are never in nature two beings that are perfectly alike.” But objects of the senses, objects found in nature, must first be given to us in space. Therefore, “the difference in place already makes the multiplicity and distinction of objects as appearances not only possible in itself but also necessary.”
The issues at the center of the debate between Leibniz and Kant are still with us—though they sometimes manifest themselves in different ways. For example, in a recent book, A Universe from Nothing: Why There Is Something Rather than Nothing (Free Press, 2012), the physicist Lawrence Krauss takes up what Leibniz considered the fundamental question of metaphysics. Krauss proposes that normal “material” objects of the universe are manifestations or arrangements of quantum fields and that it is possible for a quantum field to be arranged in such a way that no ordinary physical objects exist, but that this state is deeply unstable and most likely to go from a “null-object” state to a state with objects. Hence, something from nothing. Writing recently in the New York Times, the philosopher David Albert raises some pointed questions for Krauss: Where do the quantum fields come from? Why are the laws of nature what they are? And, still, why is there a world of quantum fields at all? None of these questions are answered by Krauss, and without answers to these questions it would seem philosophically presumptuous to claim to have answered the fundamental question of metaphysics. But even Albert’s questions themselves presuppose Leibniz’s principle of sufficient reason—that there is a reason for everything. For his part, however, Kant had argued that the principle of sufficient reason is valid only in the realm of experience and that it does not even have meaning when applied to things outside the world of sense (and prior to the origin of the world). In other words, the proper metaphysical view is that natural science, conceived as an inductive and deductive enterprise based on sense experience, can never give us an answer to the question why is there something rather nothing? But neither can rationalist metaphysics. In the end, Kant counsels philosophical humility. 1
1 But who wants to be humble? Consider the following concluding argument. Despite its tendency to pretentious nonsense,
(1) Metaphysics is better than nothing.
Colleagues in the School of Natural Sciences, however, certainly believe that
(2) Nothing is better than empirical science.
Colleagues in the School of Mathematics, meanwhile, surely believe in the transitivity relation:
if a > b and b > c, then a > c.
(3) Therefore, metaphysics is better than empirical science.
Despite the apparently rock-solid deductive form of this argument, colleagues in the School of Social Science likely see in it merely disciplinary posturing. As a philosopher, however, I must sadly (and humbly) admit that, even if the conclusion is true, the argument itself commits the fallacy of amphiboly, for it trades on the ambiguous meaning and grammar of “nothing” and expressions involving it. I suspect that any proposed answer to the question “Why is there something rather than nothing?” will flirt with other ambiguities attached to nothingness.