Logic

I want to know if the structures of logical arguments changes by logicians over time (like scientific theories might change in light of new evidence)? I also wanted to know if there might also occur changes in existing logical fallacies (other than adding more fallacies to the list) and will it ever happen that the things that are now listed among logical fallacies might become a valid way to reason (I don't think so)?

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Logicians have developed many

Logicians have developed many different theories of how logical arguments work, and the theories have plainly changed over time. Nevertheless, many standard examples of argumentation that logicians have long regarded as valid have been remarkably stable throughout history. As a result, what has changed is not so much the underlying reasoning that logicians have sought to capture as the ways in which they have tried to capture it.

Specifically, when logicians think about deduction, they seek to capture the form of arguments, but logical form can be represented in different ways. For example, Aristotle sought to capture the reasoning behind classification--meaning arguments in which one predicate includes or excludes another predicate (or the ways in which one class includes or excludes another class). This became his theory of the syllogism.

About a century later, by contrast, other logicians sought to capture the ways in which the truth or falsity of whole propositions could entail the truth or falsity of others, independently of whether their predicates included or excluded each other--and this became the propositional logic of the ancient Stoics. In the nineteenth century, much of this early ground was revisited and expressed in powerful new systems of symbolic logic, and though these new symbolic systems differed from what had gone before, they still recognized the validity of an Aristotelian syllogism like, "If all men are mortal, and if all Athenians are men, then all Athenians are mortal." What was different was the ways in which the new symbolic systems represented logical form, but the arguments being expressed were often the same.

In general, as the history of logic has unfolded, ever larger stretches of valid reasoning have been brought within the scope of formal logical systems. Admittedly, logicians sometimes differ over the validity of special cases, and these disagreements often animate their debates about which sorts of systems are best. (The American logician Willard Van Orman Quine once coined the term "deviant logic" for systems that contradicted the symbolic system that he favored.) Nevertheless, when it comes to validity, there is still broad agreement on a great range of valid arguments, and so this situation seems different from that of 17th-century physics--where the theory of heliocentrism, for instance, displaced the fundamentally different theory of geocentrism.

Part of the reason for this difference is that, though logicians can have all sorts of reasons for preferring one formal system over another, none of these systems seem susceptible to empirical confirmation or refutation--unlike theories of physical science. Logic and mathematics are traditionally classed as a priori disciplines, rather than as empirical ones, and if there is any empirical test of logical principles at all (as is sometimes asserted), the test is at best deep and remote.

As for fallacies, lists of fallacious arguments have been compiled since the days of Aristotle. On the whole, over the course of many centuries, this list has merely got longer--as people have discerned more and more ways in which well-meaning citizens are gulled and bamboozled into believing something false. Many new fallacies actually turn out to be little more than novel variations on the old ones. But it seems fairly evident that the list of common fallacies has expanded. A landmark addition to the list, by the way, came from Jeremy Bentham in his Book of Fallacies, which appeared in England in 1824, just as public argumentation in England's Parliament was coming to play an ever-increasing role in politics.

Historically, the more public speaking has influenced politics and government, the more people have tended to study fallacies--as a means to combating them.

You've posed an excellent question, and I hope my answer is not too long.

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