WE all know the joke about assume making an ass of u and me, but what exactly is the difference between these two words and how do you know which one to use?

Sometimes it seems obvious, as in this - 'I presume you know which one to use'. That doesn't quite sound right as 'I assume you know which one to use' or does it?

Kat x

For me, 'presume' implies a level of certainty that 'assume' doesn't, and also a level of personal involvement, or subjectivity: Eg 'Doctor Livingstone, I presume' implies that Stanley knew very well it was Dr.L. If he said, 'Doctor Livingstone, I assume' it would be far less involving, more of an objective stance. Of course, there's also 'presumption' which implies a kind of uninvited barging-in on something, whereas 'assumption' is more of a logical,standing-back kind of stance.
Susiex

One possible definition:

To assume is to take it upon oneself (eg. assume office), while to presume is to take it one step too far to the point where you are taking liberties.

As Susie says, Stanley was pretty sure it was Livingston, and was making a bold assumption, rather than simply assuming that it might be Livingstone.


- NaomiM

<Added>

Of course, if it hadn't been Livingstone, he would have had a lot of egg on his face.

Ah, Katerina, you see, I'd always say 'I assume you know what you're doing'.

Presume, to me, implies prior-ness - it's something you thought before. Hence 'presumption', either meaning that you have an idea already, that you're now developing from, and also the offended kind of presumption, as in someone who presumes that you'll be grateful for their help, and so goes storming in without actually asking if you want them to.

Fowler says that

In many simple contexts where the meaning is "to suppose" the two are interchangeable. Otherwise the choice of word depends on the degree of tentativeness behind the assumption or presumption. The OED definitions are very similar. Assume is 'to take for granted as the basis of argument or action; presume is 'to take for granted, to presuppose, to count upon'. There is a faint suggestion of presumptuousness about 'presume'... In the sense 'suppose' the object-clause [i.e. what you're presuming] after 'presume' expresses what the presumer really believes, till it is disproved, to be true; that after 'assume' what the assumer postulates, often as a confessed hypothesis'

EMma

So the joke about "assume making an ass of u and me" is technically incorrect, since it should refer to presume.

As a lawyer, when I presume something I have thought about the situation in advance and knowingly decided that the facts will be taken to be x unless and until proved otherwise. It is deliberative, and pre-set. (I appreciate, though, that this is a rather specialist, technical use of the word.)

On the other hand, I think of an assumption as tending to be something I just fall into because I haven't addressed my mind to the matter. I assume it to be so because I have considered no alternative possibilties. ('Oh, sorry, I'd just assumed...'.)

In other words, I think assuming is just taking things to be a certain way without reflection. Presuming is assuming in advance (whether deliberatively, as with legal presumptions, or otherwise).

But this is probably all nonsense.

Rosy