Immanuel Kant entered the realm of speculative cosmology, which he saw as centering on the idea of the world as the totality of the causal sequence of phenomena. In this way, he addressed what he saw as a conflict between ideas about human freedom and the general explanation of causality as offered by science. The issues raised included a concern for the meaning of human freedom, an understanding of how the human mind works, and a consideration of what this meant for moral reasoning.
The speculative cosmologist attempts to extend his or her knowledge of the world by means of synthetic a priori propositions. Kant stats, though, that this procedure leads to antimonies, and these arise when each of two contradictory propositions can be proved. If it is true that speculative cosmology leads to antimonies, then it must be assumed that its whole aim is mistaken, the aim of building up a science of the world considered as the totality of phenomena:
This branch of speculative metaphysics is not, and cannot be, a science. In other words, the fact that speculative cosmology is productive of antimonies shows that we cannot make scientific use of the transcendental Idea of the world as the totality of phenomena (Copleston 286).
Kant describes four antimonies, each of which corresponds to one of the four classes of categories. The third antimony relates to free causation. Kant begins with the following thesis:
Thesis: Causality according to the laws of Nature is not the only causality from which the phenomena of the world can be derived. To explain them, it is necessary to assume another causality, causality through freedom. Antithesis: There is no freedom, but everything in the world happens solely according to the laws of Nature (Kant, Critique of Pure Reason 472-473).
Kant proves the thesis by assuming that there is only one kind of causality, namely causality according to the laws of Nature. A given event is ...