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Pure reason, in both its theoretical and practical forms, has a tendency to run into a certain sort of problem. If one thing depends on another, pure reason expects to be able to trace the dependencies back until it finds the thing that depends on nothing else. However, such an endpoint to any dependency can be found only in the noumenal realm, not in the phenomenal realm. Since the phenomenal realm is the only one we have access to, pure reason is bound to be frustrated.
When pure reason is thus frustrated, it produces "antinomies," conflicting statements both of which appear to be validated by reason. The first Critique contains the antinomies of pure theoretical reason and concludes that to resolve them, we must investigate the workings of pure theoretical reason. Similarly, we will find an antinomy of pure practical reason here that will prove ultimately beneficial since resolving the antinomy will further our knowledge.
The particular series of conditions in question here relate to the good. If an action's goodness depends on something about which it does not itself depend on, what is that? Whatever it is, let us call it the "highest good." To know well enough for practical purposes what the goodness of actions depends on can be termed wisdom. To know—or more modestly, to seek to know—on what the goodness of actions depends in a scientific sense is philosophy, as "philosophy" was understood by the ancient Greeks.
The highest good is the object of pure practical reason. We must carefully draw a distinction, though between the object of pure practical reason and the determining ground we are moved by when we are moved by pure practical reason. The ground of pure practical reason is not the achievement of the highest good. It cannot be, for if it were, one's motivation to follow the moral law would depend on whether or not one cared about the highest good. It is not acceptable for obedience to the moral law to be contingent in that way. Rather, the ground of pure practical reason can only be that of dutifully following pure practical reason.
Kant's uses the term "dialectic" to connote neither a "logical argument" nor a "discussion." His dialectics are arguments that go astray because of some wrong presupposition. Or rather, they are arguments that come in pairs, both of which go astray in opposite ways because of the wrong presupposition. The Dialectic section will then seek to remove the presupposition in order to generate a more justified conclusion about the topic at hand. In this sense, the dialectic is like a discussion, where the two wrong arguments that contain a grain of truth are the two participants. Hegel and Marx model their dialectics after Kant's, where the partial truths of the "thesis" and the "antithesis" are reconciled by the "synthesis" of the two.
If the dialectic of pure practical reason is to run as Kant intends, we must be able to make sense of his distinction between the "object" and the "determining ground" of pure practical reason. That one can create a verbal distinction is clear, but what it might mean is not so clear. Unless we know what Kant means by these terms, we cannot begin to say whether he is right about what object and what determining ground are appropriate to pure practical reason.
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