But he recognized that the tool one had to use was reason itself. Thus, what was called for was a kind of self-critique . When these conditions are weakened and one allows probability as a basis for knowledge, the obstacles to knowledge are significantly lessened, at least to the extent that extreme skepticism is avoided. Pure reason limits what any human can think , i.e. it establishes the conditions necessary for thinking consistently. Theoretical reason limits what a human can know through observation and the application of mathematical laws describing what is observed.
Theoretical reason is reliable provided one remains within the bounds of human observation.
Thus, the forms of reason are legitimate only within their proper domains. Kant also observed that humans are held responsible for certain actions. But this makes sense only if those actions are freely chosen . Thus, we must be free in the sense of having a free will. Note, however, that this is a conclusion reached not by scientific, theoretical reason.