When people mix up the words “assume” and “presume,” it’s not because they’re similarly spelled. Rather, it’s because they’re close in meaning, since both can mean “suppose.”
But there’s a subtle difference in how these terms are defined, so you should be careful not to confuse them in academic writing or other situations where precision is important.
Assume (Take for Granted)
If we “assume” something, we are supposing it to be true without evidence. For example, if we help ourselves to something at a deli counter in a shop because we think it’s a free sample, only to then get the shop owner asking us if we plan to pay for it, we might say:
Sorry! I assumed it was a sample!
We’d then probably buy twice as much as we need of said product, because we’re prone to overcompensation. Regardless, the word “assume” implies jumping to a conclusion.
A second meaning of “assume” is to “take up” or “adopt” something, such as a duty at work:
When Boris retires, I will assume his responsibilities.
Or a physical position, like when police frisk a suspect:
As they made the arrest, the police told him to “assume the position.”
This use of “assume” is less common, but it’s worth keeping in mind in case you come across it anywhere (especially in professional settings).
Presume (Guess Based Upon Evidence)
The word “presume” means to make a guess based on available evidence. The most famous example of this is probably when the explorer Henry Morton finally found David Livingstone in the African jungle. As a greeting, Morton then said:
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Dr. Livingstone, I presume?
“Presume” is the correct term here because David Livingstone was the only white European known to be in that part of Africa at the time, so Henry Morton could be fairly sure that he’d found the right person when he came across a white man other than himself!
A secondary meaning of “presume” is to do something without permission, such as in:
Do not presume to tell me what I know about grammar!
This use is related to the word “presumptuous,” which describes failing to observe the limits of acceptable behavior.
Assume or Presume?
Since the secondary meanings of these words are very different, it should be easy to avoid confusions. It’s when they’re used to mean “guess” or “suppose” that it gets tricky.
One helpful memory aid is the joke that “assume” makes an “ass” out of “u” and “me,” since this spells “assume” as well as warning against the hazard of making assumptions! Remember:
Assume = Take for granted without evidence
Presume = Guess to be true based upon what is known