The concept of causality as natural necessity, as distinguished from the concept of causality as freedom, concerns only the existence of things insofar as it is determinable in time and hence as appearances, as opposed to their causality of things in themselves. Now, if one takes the determinations of the existence of things in time for determinations of things in themselves (which is the most usual way of representing them), then the necessity in the causal relations can in no way be united with freedom; instead they are opposed to each other as contradictory. For, from the first it follows that every event, and consequently every action that takes place at a point of time, is necessary under the condition of what was in the preceding time. Now, since time past is no longer within my control, every action that I perform must be necessary by determining grounds that are not within my control, that is, I am never free at the point in time in which I act. Indeed, even if I assume that my whole existence is independent from any alien cause (such as God), so that the determining grounds of my causality and even of my whole existence are not outside me, this would not in the least transform that natural necessity into freedom. For, at every point of time I still stand under the necessity of being determined to action by that which is not within my control, and the series of events infinite a parte priori which I can only continue in accordance with a predetermined order would never begin of itself: it would be a continuous natural chain, and therefore my causality would never be freedom.

From the Chapter on The analytic of practical reason, The Second Critique, Immanuel Kant