Presuppose, presume, assume, postulate, premise, posit are comparable when they mean to take something for granted or as true or existent especially as a basis for action or reasoning.
Their corresponding nouns presupposition, presumption, assumption, postulate, premise, posit when they denote something that is taken for granted or is accepted as true or existent are distinguishable in general by the same implications and connotations as the verbs.
Presuppose and presupposition, the most inclusive of these words, need not imply dubiousness about what is taken for granted. At the one extreme they may suggest nothing more than a hazy or imperfectly realized belief that something exists or is true or an uncritical acceptance of some hypothesis, in either case casting doubt on what is taken for granted.
At the other extreme the terms may be used in reference to something that is taken for granted because it is the logically necessary antecedent of a thing known to be true or the truth of which is not presently in question.
Presume and presumption may imply conjecture, but ordinarily they carry the implication that whatever is taken for granted is entitled to belief until it is disproved. Therefore one presumes only something for which there is justification in experience, or which has been shown to be sound in practice or in theory or which is the logical inference from such facts as are known.
Assume and assumption stress the arbitrary acceptance as true of something which has not yet been proved or demonstrated or about which there is ground for a difference of opinion.
Postulate, either as verb or noun, differs from assume or assumption in being more restricted in its application and more exact in its implications. One can assume or make an assumption at any point in a course of reasoning, but one postulates something or lays down a proposition as a postulate only as the groundwork for a single argument or for a chain of reasoning or for a system of thought. Postulate, therefore, has reference to one of the underlying assumptions, which are accepted as true but acknowledged as indemonstrable and without which thought or action or artistic representation is impossible because of the limitations of human knowledge or of human reason or of art.
Premise is often used as though it were identical in meaning with postulate. Premise, the noun, in logic denotes a proposition, or one of the two propositions in a syllogism, from which an inference is drawn. In more general use it may refer to a proposition which is the starting point in an argument.
But a premise is not a proposition that is frankly an assumption, as a postulate often is; it may have been previously demonstrated or it may be admitted as true or axiomatic, but it is always advanced as true and not as assumed.
Premise, the verb, means to lay down as a premise or to base on or introduce by a premise or other pertinent matter and usually refers to the broader rather than to the technical meaning of the noun.
Posit, as noun and verb, comes close to postulate in implying the laying down of a proposition as a base for an argument, a line of reasoning, or a system of thought, but it may differ in suggesting subjective and arbitrary grounds rather than, as postulate regularly does, objective and rational grounds for selection of the proposition, but even when it connotes actual falsity it remains very close to postulate .