posit has 7 distinct synonyms spread across 4 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
Synonyms, antonyms and the broader and narrower relations for posit come from Princeton University’s WordNet. Full sources and licences.
put before
“I submit to you that the accused is guilty”
take as a given; assume as a postulate or axiom
“He posited three basic laws of nature”
(logic) a proposition that is accepted as true in order to provide a basis for logical reasoning