To assume is to take something as true without evidence — you simply proceed as though it is. To presume is to suppose it on reasonable grounds. Presumption has a basis; assumption need not. "I presume you have read it" implies you had reason to think so.
The core difference
Both suppose something unverified. Only one implies a basis for it.
- assume — take as true without proof: I assumed the meeting was cancelled.
- presume — suppose on reasonable grounds: presumed innocent until proven guilty.
The legal phrase
Presumed innocent uses the word precisely: the presumption rests on a principle, not on a guess. "Assumed innocent" would suggest a baseless supposition, which is the opposite of the point.
The second sense of presume
Presume also means to act with unwarranted boldness — presuming on someone's hospitality, and the adjective presumptuous. That sense has no counterpart in assume, and it is why "I would not presume" is a formula of politeness.
Frequently asked questions
Which one implies evidence?
Presume — it supposes something on reasonable grounds. Assume needs no basis at all.
Why is it "presumed innocent"?
Because the presumption rests on a legal principle rather than a guess, which is exactly what presume conveys.
What does presumptuous mean?
Acting with unwarranted boldness — from the second sense of presume, which assume does not share.