§3. Theoretical reason and its presupposition.
At this point the argument takes a surprising turn. Kant bases his case, not merely on the nature of practical reason, but on the nature of theoretical reason.
'We cannot possibly conceive of a reason as being consciously directed from outside in regard to its judgements.' If a rational being were conscious of any such external influence, he would regard his judgements as determined, not by reason, but by impulse. Reason must -- if it is to be reason at all -- regard itself as the author of its own principles independently of external influences.
This is a strong argument, though it is seldom used in discussions about freedom. It applies most obviously to a judgement which is the conclusion of an argument. If every judgement is determined solely by previous mental events and not by rational insight into a nexus between premises and conclusion independent of temporal succession, there can be no difference between valid and invalid inference, between reasoning and mere association, and ultimately there can be no truth. In that case determinism itself could not be accepted as true, nor could the arguments in its defence be accepted as valid.
Kant seems to be correct in saying that any reason which is conscious of itself as reason must regard itself as reasoning (or as forming its own conclusion) in accordance with its own rational and objective laws or principles, and not by the influence of any external cause or bias. This remains true even although it may be reasoning about something given it from without, as, for example, so-called sense-data. That is to say, reason must regard itself as the author of its own principles and as capable of functioning according to these principles independently of external influences. This means, in Kant's terminology that reason must regard itself as free, both negatively and positively, in the act of reasoning.
§4. Practical reason and its presupposition.
If Kant's argument holds of theoretical reason, it holds equally of practical reason - that is, of a rational will or of reason as exercising causality. Here too a rational agent as such must in action presuppose his rational will to be the source of its own principles of action and to be capable of functioning in accordance with these principles. In other words he must in action presuppose his rational will to be free both negatively and positive -- that is, to be free from determination by desire and free to obey its own rational principles. To say this is to say that a rational agent can act only on the presupposition that he is free: he must act under the Idea of freedom. This is the doctrine which we set out to establish and from which the principle of autonomy is said to follow analytically.
This doctrine need not be based merely on an inference from theoretical to practical reason -- though Kant may think that it is. It may rest also on the same kind of rational self-consciousness as does the previous argument. We may perhaps say that our insight into theoretical reason is also an insight into reason as such and consequently must cover practical reason as well; but the same insight into reason as such is surely found again in our insight into practical reason, and indeed must be found again if our conclusion is to be justified.
I do not want to make too much of this, but I believe Kant to be saying more than that in acting we necessarily conceive ourselves to be free. Action is not a blind something which is preceded and succeeded, or even accompanied, by thought. Action is as intelligent and as rational as thinking. What distinguishes human action from animal behaviour, and still more from physiological functioning or physical movement, is that we will in accordance with principles. I take Kant to be saying that a rational agent can act, just as he can think, only on the presupposition of freedom: he must think and act as if he were free. The presupposition of freedom is as implicit in his acting as in his thinking; and unless we can act on this presupposition there is no such thing as action, and there is no such thing as will. As Kant himself puts it, 'The will of a rational being cannot be a will of his own except under the Idea of freedom'. Human action cannot differ from animal behaviour merely in being accompanied by a conception of freedom: if it differs at all, it must differ by being itself rational. A rational agent must will his actions under the Idea of freedom, just as he must will his actions as instances of a particular principle or maxim.
No doubt there are many today who will be content to deny that there is any such thing as action or as will, if by this is meant rational action and rational will. This view can be reasonably held, as Kant recognises, if we take up the standpoint of observers. But if we regard thinking from the same external point of view -- and it is arbitrary not to do so -- we shall equally be compelled to deny that there is any such thing as thinking. There are some who are willing to accept even this conclusion; but if they do, it seems foolish to try to convince others of it by argument.
H. J. Paton
The Categorical Imperative: A Study in Kant's Moral Philosophy, 6th ed.