Much of this site’s content centers around the attempt to put together reasonable arguments in support of one position or another. We haven’t really spent much time exploring what ‘reasonable argument’ are, however, and one of the quickest ways to end a potentially constructive conversation is to let basic terms go unexamined. It might seem like an understanding of logic and reasoning should be common sense, but within the philosophical arena, there are fundamental differences about the very nature of logic and reasoning that aren’t just academic hand-wringing. Those differences often spill out into people’s everyday discourse – as do the errors that drive logic professors crazy. So what I’d like to do is start a sort of primer to basic structures of logic, and touch on some of the related issues.
What I’m discussing here is very basic, formal deductive logic. It’s formal not in the sense that it wears a three-piece suit, but in that it’s concerned with the form, or structure, that an argument takes – how its parts fit together, and what does and does not follow from given pieces of information. A logician will often work with symbols rather than actual arguments to keep this emphasis clear. It’s deductive because it works from given information to determine what other facts absolutely must be true – there are no shades of gray or degrees of probability. Of course, not every argument that one encounters will fit neatly into a particular formal structure or be amenable to a strict yes-or-no evaluation. Much of the reasoning we do in everyday life is of the inductive variety, which factors degrees of probability into the mix. But many of the underlying principles are the same, which makes the study of formal deductive logic worthwhile.
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