Speaking of definitions:
theory
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noun [C/U]
something suggested as a reasonable explanation for facts, a condition, or an event, esp. a systematic or scientific explanation
Adele took a course in modern political theory.
I have a theory (= an opinion) about why everybody in the city is in such a hurry.
They could, in theory (= possibly), have been paid twice if someone hadn't caught the error.
theoretical
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adjective
based on theory or on possibilities
The president does not want to answer any theoretical questions.
We can just as easily call it the:
approach, argument, assumption, base, basis, code, codification, concept, conditions, conjecture, doctrine, dogma, feeling, formularization, foundation, grounds, guess, guesswork, hunch, hypothesis, idea, ideology, impression, method, outlook, philosophy, plan, plea, position, postulate, premise, presentiment, presumption, proposal, provision, rationale, scheme, shot, speculation, stab, supposal, suppose, supposition, surmise, suspicion, system, systemization, theorem, thesis, understanding
of evolution.
What we can't call it with mathematical certainty is:
THE FACT OF EVOLUTION AS THE ONLY ANSWER TO THE QUESTION OF MAN'S EXISTENCE.