Assume vs. presume

Assume and presume both mean to take something for granted as true (among their many other definitions). The difference is in the degree of certainty. A presumption is usually more authoritative than an assumption. To presume is to make an informed guess based on reasonable evidence, while to assume is to make a guess based on little or no evidence.

Examples

In this example, presume is the better word choice because the speaker’s guess is based on irrefutable evidence:

I told the host, whom I presume to be the owner judging from the picture on the website, that I had reservations. [SF Gate]

And in this example, the speaker states that he doesn’t know, which makes his guess a clear-cut assumption:

I assume police have at least interviewed them. I don’t know if they have or not. [Express]

19 thoughts on “Assume vs. presume”

  1. There is a closely related use of “assume” in which it can never be replaced by “presume”. It’s an absolute distinction, not a matter of degree, as in your examples.

    This is where you assume something “for the sake of argument”, use the assumption to draw some conclusion, and then “discharge” the assumption. After you have discharged the assumption, you don’t know that the conclusion is true, but you do know that the implication,

    “if the assumption is true, the conclusion must also be true”

    (or “assumption => conclusion”, in symbols)

    is true. You can store this implication for future use so that, if you later discover that the assumption is indeed true, you will be able to immediately rely on the conclusion also being true, without having to repeat the previous argument.

    When you make assumptions in this sense, you don’t have to be at all confident that they
    are actually true, so your analysis explains why “assume” rather than “presume” is the natural choice for this meaning. But, if you do it properly, this sort of reasoning should never lead to false beliefs, so it is a distinct usage from your main account of “assume”.

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  2. When you assume, you usually make an “ass” out of “u” and “me”. That’s exactly what happens when you don’t have enough information/evidence to make a presumption.

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