tickitytak wrote:How do we know reason is reasonable?
There are many different logical systems with their own system of rules. The most obvious ones that come to mind are Boolean algebra (aka propositional calculus), intuitionist logic, paraconsistent logic, predicate calculus, modal logic (of which there are dozens of competing systems), and many different many valued logic or fuzzy logics. Some of these logics are in direct competition over which rules of thinking are correct.
Non formal arguments can unwittingly range across several logics. Mostly folk argue within the propositional calculus, predicate logic and modal logic and there are some standard rules like modus ponens. Whilst arguing informally it is sometimes helpful to be explicit as to which logic an argument is supposed be formed in.
Which are reasonable? Well they are each reasonable according to who you speak to. As for how do we know some rule is reasonable that usually comes down to self evidence. We look at a rule and cannot think of way it can possibly be false. Just as the 1 minus 1 = 0: whichever way we look at if we have 1 and take 1 away that leaves nothing. Rules like “If p then q, and p is true therefore q� is also self evidently correct. These rules show the limits of how we think. It is not possible to provide further evidence or supporting argument for them. You either get them or you don’t....and you are supposed to get them because they are deemed to be basic rules of thinking. Someone may try to insist that “if p then q, and p is true the q is false� but that just don’t make sense.....I hope.
Other rules - depending on thre system you are working in - are controversial to varying degrees. There are also various
paradoxes and unintuitive results drawn from otherwise self evidently correct rules that logicians shrug their shoulders and accept. That there are so many logics is perhaps indicative that some logicians at some point are being unreasonable, and the paradoxes maybe a hint that we have to figured out all the rules yet. the idea that there should be one universal logic underlying all logics is appealing.
There are also many well understood fallacies which are really a combination of formal and less formal argument fallacies. Again their unreasonableness is usually self evident. I'll also suggest that often the problem in less formal debate comes down to a collection of less formal fallacies like "poisoning the well" and "no true Scotsman".
