If you expostulate with someone, you express strong disagreement with or disapproval of what that person is doing.
If you postulate something, you assert that it is true without proof; therefore, it can be used as a basis for argument or reasoning—even though there is no factual basis for the assumption.
the act of expressing earnest opposition or protest
(logic) a proposition that is accepted as true in order to provide a basis for logical reasoning
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