I assume you are reading this because you presume my blog will be an accurate source of information about word usage. I can only assume that because I don’t know who you are nor why you are really reading this post. It’s just a guess. You, on the other hand, would presume my expertise, since I have identified myself as a professional editor. Until proven otherwise, a presumption carries more weight.
In everyday speech, “assume” and “presume” are often used interchangeably, but that’s incorrect. According to the website Grammarist, while both words can mean to take something for granted as true (among their other definitions), the difference is in the degree of certainty. A presumption is more authoritative; to presume is to make an informed guess based on evidence, while to assume is to guess with little or no evidence.
Take these examples:
Looking at the photo, I presumed the band had five members, but in concert it turned out to be a quartet.
I assumed the reporter had interviewed the band, although I did not know that he did.
In the first, presume is the better word to use, because the guess is based on irrefutable evidence. In the second, the guess is clearly a guess.
Got it? Whenever you’re having trouble finding the right words in your own documents, it’s time to hire an editor.