proper and to be expected as a matter of course; right and inevitable according to reason

to assume that something must be so without having any factual evidence; to make subjective assumptions or conjectures

To assume something must be so based on subjective reasoning; to take as a matter of course even if the facts are unproven.

to assume something is true based on one's own imagination; to take something as given; to think that it should be so as a matter of course