The Metaphysics of Morals by Immanuel Kant Chapter 3 Page 31

though it may possibly without contradiction be reconciled with another that is sufficiently established, yet entangles us in a perplexity which sorely embarrasses reason in its theoretic employment. This duty, however, belongs only to speculative philosophy. The philosopher then has no option whether he will remove the apparent contradiction or leave it untouched; for in the latter case the theory respecting this would be bonum vacans, into the possession of which the fatalist would have a right to enter and chase all morality out of its supposed domain as occupying it without title.

We cannot however as yet say that we are touching the bounds of practical philosophy.

For the settlement of that controversy does not belong to it; it only demands from speculative reason that it should put