What is the difference between assume and presume? In looking that up, they are cited as synonyms. Is the technical difference that you know something about what you are assuming, but not with what you are presuming?
The entry for presume at AHD has details on the synonyms of presume
presume, [...] assume These verbs signify to take something for granted or as being a fact. To presume is to suppose that something is reasonable or possible in the absence of proof to the contrary: “I presume you're tired after the long ride” (Edith Wharton). [...] To assume is to accept something as existing or being true without proof or on inconclusive grounds: “We must never assume that which is incapable of proof” (G.H. Lewes).
See the entry (link) for the other synonyms and examples of them, too.
Somehow, though, they have a slightly different feeling to them. There is something more assertive about presuming than there is about assuming. When we say someone is "presumptuous," we mean he has gone a bit too far in his assumptions (and who does he think he is?!) By contrast, you never hear of anyone being "assumptuous," if that is even a word. We might say, "She assumes incorrectly," but somehow it is not as offensive as if she were to presume incorrectly.
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I do agree with you, WM, and yet in those 2 AHD examples above, I could see using either "assume" or "presume." My daughter reminded me of the quote: "Dr. Livingstone, I presume?" Somehow "assume" just doesn't work there.
Interesting, WM, that there isn't an "assumptuous," you are right. However, according to OED, there is an "assumptious," which is a "rare" adjective, meaning "given to assumption, assuming. Hence a'ssumptiousness, tendency to take too much upon oneself."