Difference between assumption and presupposition

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What is the difference between an assumption and a presupposition? The online dictionaries give very similar definitions, yet I was told that there's a crucial difference between the two terms - especially in the context of philosophy of language.
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Nik the Greek wrote on 02 Dec 2004:
[nq:1]What is the difference between an assumption and a presupposition? The online dictionaries give very similar definitions, yet I was told that there's a crucial difference between the two terms - especially in the context of philosophy of language.[/nq]
I don't remember what I might have read about the difference in philosophy of language books, but one clear difference is that a presupposition exists before X and an assumption can be created after or as a result of X. It is also clear that we bring assumptions to bear on all Xes, and they can be called presuppositions, eg, that the sun will rise in the east again tomorrow, that humans are social animals, and that 2+2=4. Language philosophy is filled with this kind of technical distinction.
An assumption is also (W3NID): "the minor or second premise in a categorical syllogism".

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[nq:1]What is the difference between an assumption and a presupposition? The online dictionaries give very similar definitions, yet I was told that there's a crucial difference between the two terms - especially in the context of philosophy of language.[/nq]
Philosophy I wouldn't know. But in a logical exercise, you can deliberately assume anything as a basis. I can assume that 1 is greater than 100 and construct a theory that remains consistent with that. The results of that reasoning will presuppose that my basis was 1>100.
Or I can assume something about what I don't know about, for instance that the sun emits heats because millions of little devils inside are running in a big wheel, without presupposing anything like that.

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[nq:1]What is the difference between an assumption and a presupposition? The online dictionaries give very similar definitions, yet I was told that there's a crucial difference between the two terms - especially in the context of philosophy of language.[/nq]
There's a difference, all right.
"Assumption" is a general term without any specific meaning in linguistics or philosophy. That is, it means anything you want it to mean; it's just the ordinary nominalization of the verb "assume".
"Presupposition", on the other hand, is a technical term widely used in both linguistics and philosophy of language, with a very specific sense.

A presupposition is a proposition (statement, sentence) that must be understood to be true in order to make sense of a given chunk of language. All kinds of language chunks have presuppositions. 'The' has a presupposition of existence, for instance for someone to talk meaningfully about 'the man with two heads' it has to be true there is some man that has two heads, and that the speaker is aware of that, and expects the listener to be, as well.
There's a whole class of verbs called "factive" verbs whose claim to fame is that they presuppose their complement. This is what makes the second sentence below weird:
Bill doesn't believe the world is flat.
#Bill doesn't know the world is flat.
'Know' is a factive, which means the speaker must be committed to the proposition that the world is flat.
The standard test is that a presupposition doesn't get negated when you negate the whole sentence. Thus if
The present King of France is bald.
is false (and who would want to assert it was true?), then its negation,

The present King of France is not bald.
must be true. But negation doesn't touch presuppositions, so both of them presuppose that there exists a present King of France, and thus neither of them can really be true. This is the problem (currently called "presupposition failure") that led among other things to the development of ternary logic and Polish notation by Lukasiewicz in the 1920s.

There's an immense literature on presuppositions; Google Scholar produces 16,200 hits for 'presupposition', one of which from the first page is the complete chapter on Presupposition from the Handbook of Logic and Language at
http://www.ling.uni-potsdam.de/~jaeger/praesupp ws0203/beaver96presupposition.pdf

If that seems overkill, a short list of some of the kinds of presuppositions (and entailments, a different kind of propositional correlatives) and the kinds of language chunks that trigger them, from my Grammar and Meaning class, can be found at
http://www.umich.edu/~jlawler/presuptrig.pdf
-John Lawler http://www.umich.edu/~jlawler U Michigan Linguistics Dept "You can only find truth with logic if you have already found truth without it." G.K. Chesterton

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Thank you so much for taking the time to explain it so thoroughly. I think I've got it now.

Difference between assumption and presupposition

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