Assume vs Presume

‘Assume’ and ‘presume’ are both used in a similar way. They are both verbs. Their shared definition means to suggest that something is true, the same way words such as ‘suppose’ or ‘guess’ are used. However, the two words imply different things and they are used at different times.

‘Assume’ means to make a guess based on weak evidence.

“I assume that the red fingerprints on the wall have nothing to do with your red fingerpaint?”

In this case, the above sentence is sarcasm, so it’s not supported by any evidence, because the evidence points in the other direction.
“While it startled him, he assumed the cat made that strange noise in the other room and went back to his book.”

To presume, on the other hand, is to make a guess based on strong evidence.
“I see you bought a few pairs of chopsticks and a lot of Chinese cookbooks. I presume you’re going to start trying some new recipes?”

So, which of the two you use largely depends on how confident you – or the person saying it – are in the statement.
“She assumed that the cat was a cross between the Ragdoll breed and an Egyptian Mau.”
“She presumed that the cat was a cross between the Ragdoll breed and an Egyptian Mau.”

In the first sentence, she isn’t so sure about the cat’s breed. In the second, she is.

It can also indicate expertise in a subject. A cat enthusiast might make an assumption, but a judge in a cat show who is specifically trained to recognize a cat’s breed might be able to presume.

‘Presume’ is also a more formal word than ‘assume’, and it’s more likely to be found in those settings. That is mostly because of the word’s association. Often, in formal settings, you’re expected to have more evidence for something than in casual conversation, so it’s used more often there.

Some native speakers will use the words differently. There are some schools of thought that say that ‘assume’ should be used for something that is happening immediately or has happened, and that ‘presume’ is for future situations.
“I assume that you are taking business classes this year?”
“I presume that you will be taking business classes next year?”

This is not currently grammatically correct, but it may explain some sentences with odd usages. English is also a rapidly evolving language, so it’s entirely possible that it could become a regular use of those words. For right now, it’s best to stick with what’s correct right now.

‘Assume’ has a different, unrelated meaning. It is also used in the sense of ‘to take on’. It’s especially used in the phrase ‘assume responsibility’.
“I am going to assume responsibility for the Human Resources Department.”
“I am going to take on responsibility for the Human Resources Department.”
“He assumed a disguise.”
“He took on a disguise.”

‘Presume’ has another meaning that’s closer to its current meaning. You might see sentences like this:

“He wouldn’t presume to come into my house while I’m asleep.”

In some cases, ‘presume’ can mean stepping out of bounds. The above sentence means something like this:
“He wouldn’t overstep his boundaries by coming into my house while I’m asleep.”

This meaning of the word is often used to describe arrogance, or someone who thinks they can get away with something that’s blatantly not okay.

In short, ‘assume’ is to make a statement that you’re not sure is true. To presume is to make a statement that you’re confident is true. ‘Assume’ can also mean to take something on yourself, and ‘presume’ can mean a kind of arrogance.
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