'Nothing comes from Nothing. Nothing ever could.'
This is probably one of the crucial foundations of rationality. Any complex idea can be critically questioned by asking what are its fundamental assumptions and axioms. This allows one to understand the hierarchy of concepts that build and hold a structure upright. Not everything can be valid under all circumstances. It is the process of applying thought to understand the conditions under which validity is preserved and the order of magnitude of the propagation of the importance of "hierarchically simpler" (fundamental) ideas i.e. which ideas are more or less important under different conditions of validity, is what can be called critical thinking.