A proposition exhibits matters of fact which both are and are not the case.

The entirety of true propositions is the the entirety of natural science. Philosophy is not a natural science, the aim of philosophy is to clarify thoughts logically. That makes it an activity, not a doctrine. Essentially, a philosophical work explains, so philosophy does not result in 'philosophical propositions', but rather in their clarification. It should, as it were, put thoughts in focus.

Psychology is no more closely related to philosophy than any other natural science. Theory of knowledge is the philosophy of psychology. Does not my study of symbolism correspond to the study of thought processes, which philosophers used to consider so essential to the philosophy of logic? Only in most cases they got entangled in unessential psychological investigations, an analogous risk with my method as well.

Darwin's theory has no more to do with philosophy than any other hypothesis in natural science.

Philosophy delimits what can be discussed in natural science. It delineates what can be thought; and, in doing so, what can not. It marks the border to what cannot be thought by working outwards from what can be thought. It will signify what cannot be said, by presenting clearly what can be said. Everything that can be thought at all can be thought clearly. Everything that can be put into words can be put clearly.