A trick to using malapropism is to ensure that the two words (the original and the substitute) sound similar enough for the reader to catch onto the intended switch and find humor in the result.
Positivism is a philosophical theory stating that positive knowledge is based on natural phenomena and their properties and relations. Thus, information derived from sensory experience, interpreted through reason and logic, forms the exclusive source of all authoritative knowledge.
by Matt Slick. Relativism is the philosophical position that all points of view are equally valid, and that all truth is relative to the individual. This means that all moral positions, all religious systems, all art forms, all political movements, etc., are truths that are relative to the individual.