Part I: General Considerations (Overview)
- Hume’s Skepticism concerning the power of intellect/Naturalism concerning knowledge and action
- Hume’s reorientation of metaphysics: from rational theism to naturalistic description of human understanding
- Kant’s understanding of Hume’s challenge for metaphysical thinking
- Partial response to the challenge already available in geometry and physics
- Model of the mind required to understand the kind of cognition possible in geometry and physics provides clue to how metaphysical cognition is possible
Hume’s skepticism concerns the supposed intellectual powers of the human mind. It does not concern our common sense view of the world and our general sense of our abilities and methods for coming to know more and more about the world. Here, his naturalistic bent shapes things in decisive ways. Metaphysics becomes an investigation of human nature that is carried out by the same limited capacities for knowledge that we make use of when we investigate other aspects of nature. It can have an unsettling effect on the mind to gain a deeper sense of its own limitations in all of these areas. For the most part, however, nature takes over and restores the confidence we need in order to act in accordance with those very same customs or habits that we bring into question when we are engaged in speculative philosophical thought experiments (like those that Descartes takes us through in the Meditations).
Metaphysics cannot be, from Hume’s perspective, a non-empirical science of things-in-themselves that exist indepedently of our experience of them. It cannot deliver scientific knowledge of God, the world as a system of substances, and the intellectual soul. The metaphysics of theism–which we see just as much in moderns like Descartes, Spinoza, Leibniz, and Locke as we do in medieval scholastics following Aquinas–cannot be invoked in defense of itself and, then, in defense of our common sense view of the world and our general trust in the reliability of our capacities and methods for coming to know more and more about the world. The best that metaphysics can do is survey the human understanding and give us some insight into our own nature and limitations that enables us to methodize and correct the knowlege we gain through common experience and custom.
Kant does not follow Hume in his view of the mind, or in his use of naturalism in defense of itself and in relation to our common sense views (here, the naturalist appears to be in no better position than the theist). He is impressed, however, with the challenge that Hume poses concerning metaphysical knowledge. Kant agrees that distinguishing metaphysics from the merely logical analysis of concepts requires showing how we can legitimately reason about real existence and matters of fact–i.e., about existing things and their powers, capacities, or characteristics. It is clear that doing this requires us to go beyond ideas of possible things and considerations of logical consistency. We need to be able to think about real relations between actual things in ways that maintain the connection between our thought and reality (e.g., there is no effect without a cause; therefore, the real event of this billiard ball coming to rest, considered as a real effect, requires that there be some real cause). We cannot transform these relations and things through our thinking to such a degree that we are treating them as merely logical relations between possible things.
So, the question for Kant concerns how to respond to the challenge for metaphysics presented by Hume’s insight concerning reasoning about matters of fact. The answer Kant comes up with involves seeing a similar challenge confronting our thinking when it comes to mathematics and to mathematical natural science (i.e., general physics). Since we tend not to have the same worries about the legitimacy of these endeavors that we have about metaphysical claims concerning God and the human soul, mathematics and physics provide a partial response to Hume’s skepticism about the powers of the human mind that is likely to be broadly acceptable. Beyond that, this response also does a couple of other important things:
- Provides a very different model of the mind than the one Hume provides
- Allows us to see salient similarities and differences between the cognition involved in geometry, in physics, and in metaphysics
If we can determine important features of human cognition that are left out of Hume’s account by appeal to geometry and physics, what we learn there might be important for understanding the possibility and the limitations of the particular modes of cognition that characterize metaphysics; i.e., general ontology (theory of substance, causality, powers), and the specific areas investigated in rational psychology (theoretical cognition of the soul), rational cosmology (theoretical cognition of the world) and rational theologoy (theoretical cognition concerning God).