Subset's of philosophy are often defined by their individual constraints. Huge areas are devoted to various assumptions such as "the bible is literally accurate" or "how people actually behave" (descriptive ethics), but the only thing they all have in common is a drive for reasonable levels of consistency. Taken to an extreme you end up with Math in all it's glory, but a complete lack of such consistency results in meaningless gibberish.

Consider the amount of effort spent to explain evil in a world created by a good, perfect, and all powerful god. Or the winding paths to define "truth" without creating a tautology etc etc. IMO, Philosophy sit's in the middle ground between religion which is happy to make inconsistent things up, math which requires self consistency, and science which must be grounded in reality.