Assumption
Etymology
From Latin "assumere" (to take up, take to oneself) — "ad-" (to) plus "sumere" (to take). The religious Assumption is Mary taken up bodily to heaven; in reasoning, an assumption is a claim you take up and carry without stopping to set it down and inspect it.
noun · /uh-SUMP-shun/
Something taken to be true without being checked, on which further reasoning rests.
What it names
The load-bearing beliefs you didn’t examine — surfacing them is often how a stuck argument gets unstuck.
Example
The plan failed on a hidden assumption: that the supplier would never raise prices.
Related idea
Compare premise: a premise is an assumption made explicit and stated as the starting point of an argument. Assumptions do their damage precisely when they stay unstated — a premise you can debate, an assumption you don’t even see.