He followed, thereby, Kant's doctrine that pure, practical reason postulates the existence of God, but he tried to transform Kant's rational faith into a speculative knowledge on which he based both his theory of science and his ethics.
Kant there reminds us that by "the theoretical path," the postulates cannot be established; yet "by the practical law that commands the existence of the highest good possible in a world, the possibility of those objects of pure speculative reason…is postulated" (5:134).
According to Kant, belief in God is "a postulate of pure practical reason": "a theoretical proposition which is not as such demonstrable, but which is an inseparable corollary of an a priori unconditionally valid practical law" (CPrR 5 122).
The most glaring example of this is at 5 122 in the Critique of Practical Reason, right in the opening line of the section "The Immortality of the Soul as a Postulate of Pure Practical Reason" – that is, right where Kant argues for the postulate of immortality.
Hence the highest good is practically possible only on the presupposition of the immortality of the soul, so that this, as inseparable with the moral law, is a postulate of pure practical reason" (ibid).. Kant's idea is not that we should imagine ourselves attaining holiness later although we are not capable of it in this life.
In the Dialectic, Kant argues that freedom "considered positively (as the causality of a being insofar as it belongs to the intelligible world)," along with the immortality of the soul and the existence of God, is a postulate of pure practical reason, belief in which is demanded (though not as a moral duty) in order to make sense of the moral law's commands (CPrR 5 132).
And still we spend a great deal of time on invisible things: some appealed to Dawkins (what the pure math of physics postulated) and others frustrated him ("fairies, ghosts, hobgoblins").
Fichte did not assert that this supreme principle was self-evident but rather that it had to be postulated by pure thought.
On this basis, he claims that it is morally necessary to believe in the immortality of the soul and the existence of God, which he calls postulates of pure practical reason.
The most important kinds of moral belief or believing-in are also what Kant calls "postulates of pure practical reason" (5: 122–134), which most notably include the immortality of the soul and the existence of God.
