Connection between Kant's defense of the belief in God and Kant's categories?

Question

Kant states in the Critique of Pure Reason (English translation) that

Thus the entire armament of reason, in the undertaking that one can call pure philosophy, is in fact directed only at the three problems that have been mentioned. These themselves, however, have in turn their more remote aim, namely, what is to be done if the will is free, if there is a God, and if there is a future world. Now since these concern our conduct in relation to the highest end, the ultimate aim of nature which provides for us wisely in the disposition of reason is properly directed only to what is moral. (A801)

The Critique of Pure Reason translation used in the wikipedia article on the matter is

All the preparations of reason, therefore, in what may be called pure philosophy, are in reality directed to those three problems only [God, the soul, and freedom]. However, these three elements in themselves still hold independent, proportional, objective weight individually. Moreover, in a collective relational context; namely, to know what ought to be done: if the will is free, if there is a God, and if there is a future world. As this concerns our actions with reference to the highest aims of life, we see that the ultimate intention of nature in her wise provision was really, in the constitution of our reason, directed to moral interests only. (A801)

I happen to find the first translation clearer (it comes from the pdf I am reading), but Wikipedia derives its analysis from the second one.

Kant had before established that it is impossible to either prove or disprove the existence of God. Wikipedia interprets this passage as arguing that thus, the next step in determining whether to accept the existence of God is to see whether it is in our interest to believe in God.

Now, Kant had previously in his epistomological expositions established categories of reason, which we humans impose onto the world. He dealt with Hume's skepticism by arguing that when we humans observe the world, we impose the category of causality (under a relation of ideas in his table of judgement) onto the world, just like a person with red glasses imposed the color red onto the world (courtesy of Sophie's world).

In both instances, Kant defends the attacks on the belief of God and of casuality, showing that both cannot be rationally proven. But then, he defends bothc casuality and the belief in God through two different mechanisms; for casuality he places it as a category of understanding, while with God he argues that it is in the interest of man to believe in God.

Now, my question is whether these mechanisms are related? Is the defense of the belief of god related to the concept of categories of understanding, or are these two completely different ideas within Kant's philosophical framework? If they are related, how? (Any relevant passages from an internet source or Kant's Critique of Pure Reason would be appreciated.)

Answer 1

I think this is a solid question, and one that might be difficult to fit into the format of an SE answer. Thus, I'll try to provide a brief sketch of what I take the longer answer to be.

First, I want to suggest that one needs to be very careful in how one uses "rational"/"reason" and "understanding"/"knowing" when speaking about Kant. In the paragraph where you write about "In both instances", you state "showing that both cannot be rationally proven", but I think this is slightly infelicitious. Kant thinks both causality and the existence of God cannot be known -- but not because they are not related to reason. Instead, it is that understanding / i.e. knowing / is the application of the categories to a thing that has been placed under the manifold of sensibility.

Thus, it makes no sense for causality to be knowable, because causality is one of the conditions of knowledge. (as in the Sophie's World account of red glasses).

Similarly, God does not fit under these categories either for Kant. Kant rejects the possibility of knowing God in this way through one of his antinomies.

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With that distinction in hand, we can turn to the quotation you provide, note the last phrase "directed to moral interests only". It has been argued by several thinkers that the Critique of Pure Reason is a propaedeutic to Kant's moral philosophy, and this is one of the passages that supports that interpretation.

Thus, to see how God and causality differ, we might turn to Kant's moral philosophy, which on a very crude sketch makes causality in the world the antithesis of rational autonomy. This rational autonomy in turn depends on the existence of God. First, God serves as a final judge meting out proportional happiness to the goodness of agents at the end of time (this is called the proportionality thesis). In the Critique of Practical Reason, Kant calls "God a postulate of practical reason" (See 5:122-134). Also see Religion from around page 96.

To put it another way, God and causality belong to two distinct realms that the Kantian moral self finds itself in. In the one realm, the self's actions are determined (causally) by what has preceded the self. In the other realm, the self uses freedom (rational freedom) to determine the course of its actions in a way that is imperceptible when that self is viewed as an object (rendered through the categories of others or even the self) but which is intimately tied to the existence of God.