“It would certainly be more satisfying to our speculative reason to solve those problems for itself without this circuit and to have put them aside as insight for practical use; but, as matters stand, our faculty of speculation is not so well off.”

This refers back to a core argument made by Immanuel Kant in his previous work, Critique of Pure Reason. Speculative or theoretical reason is flawed, at least if it is not grounded with empirical evidence and reason.

“Nothing worse could happen to these labors than that someone should make the unexpected discovery that there is and can be no a priori cognition at all. But there is no danger of this. It would be tantamount to someone’s wanting to prove by reason that there is no reason.”

A key part of Kant’s philosophy is that he believes there are certain fundamental principles that are self-evident. Along with the existence of freedom or free will (3), another of these is that there is such a thing as a priori reasoning or reasoning through intuition.

“Now there enters here a concept of causality justified by the Critique of Pure Reason although not capable of being presented empirically, namely that of freedom; and if we can now discover grounds for proving that this property does in fact belong to the human will (and so to the will of all rational beings as well), then it will not only be shown that pure reason can be practical but that it alone, and not reason empirically limited, is unconditionally practical.”

The existence of free will, which Kant takes as a postulate—or a self-evident truth—is in many ways the touchstone of Kant’s philosophy as expressed in Critique of Practical Reason. For Kant, free will is also evidence of causality and of practical reason itself.