Similar word to theory:

approach, argument, assumption, base, basis, code, codification, concept, conditions, conjecture, doctrine, dogma, feeling, formularization, foundation, grounds, guess, guesswork, hunch, idea, ideology, impression, method, outlook, philosophy, plan, position, postulate, premise, presentiment, presumption, proposal, provision, rationale, scheme, shot, speculation, stab, supposal, suppose, supposition, surmise, suspicion, system, systemization, theorem, thesis, understanding