Is there a difference between reductio ad absurdum and a straw man argument? I think I remember learning that a straw man argument was a special type of *reductio ad absurdum, *but now I can’t think of what the difference is.
Nope. The straw man fallacy is that of substituting for your opponent’s position a weaker one that he does not hold, and arguing against it instead (the metaphor is of fighting against a scarecrow instead of a real opponent).
Reductio ad absurdum is not a fallacy. It is a legitimate argument that demonstrates the truth of a position by showing that denying it leads to a false, absurd, or untenable result, or one that demonstrates the falsehood of a position by showing that the position itself leads to a false, absurd, or untenable result.
The Straw Man is also an informal fallacy, and more of rhetorical device.
??? Nametag: you seem to be describing the differences…and yet you said “Nope” that there are any differences?
Reductio ad absurdam follows the necessary logical consequences of a proposition. If I say, “We should disband the military,” it makes sense to ask, “What do we do with all of the newly unemployed people?” Eventually, as we follow the chain of logic, we arrive at a consequence that is absurd, and this serves to impugn the proposition.
It is a valid technique. In mathematics, it is seen (elegantly) in the proof that the square root of two is an irrational number.
The Straw Man fallacy is simply cheating, out and out. “Hey, when you said to dismantle the military, that means you really want the Nazis to take over.” Nope: not what he actually said. It’s one of the dirtiest forms of dirty pool.
In its purest mathematical form, a reductio ad absurdum argument is a proof that a given proposition leads to a logical contradiction. As contradictions are not allowed in most forms of logic, this serves to disprove the proposition. It has been a valid and useful part of mathematics for thousands of years. In other contexts, it might mean simply that accepting a certain proposition would lead to a result both sides agree is logically absurd.
A strawman argument is a dishonest argument technique (often called an ‘informal fallacy’, meaning that its form is not necessarily invalid, but its content is) based on replacing the position your opponent does hold with one they do not, and arguing against that new proposition.
A closer relative to a reductio ad absurdum argument is a slippery slope argument, because they both depend on following a chain of reasoning leading away from a given proposition; in mathematics, this chain of reasoning is unavoidable and implicit in the proposition, but in non-mathematical logic, the chain might be something we could agree to short-circuit before the absurd result is reached.
I think Nametag’s “nope” may be a response to " I think I remember learning that a straw man argument was a special type of reductio ad absurdum…"