Some ways of reasoning are what we call ‘invalid’. An invalid piece of reasoning, or argument, is one whose beginning assumptions do not properly support its conclusion. A fallacy is an argument form, or pattern of reasoning that is always invalid. For example, if one says ‘If I break my leg, then I’m in pain. I broke my leg, therefore, I’m in pain.’, then one reasons well, that is, validly, because this argument form ‘If P, then Q. P, therefore Q.’ is such that if the assumptions ‘If P then Q’ and ‘P’ are true, then Q is true also. However, if one argues like this: ‘If I break my leg, then I’m in pain. I’m in pain, therefore I broke my leg.’, then one reasons badly (invalidly). This is because the conclusion ‘I broke my leg’ might be false even in the case that the beginning assumptions are true. After all, maybe I’m in pain not because I broke my leg, but because I got hit square in the eye with a squash ball! The beginning assumptions therefore cannot properly support the conclusion. And this is always the case for this pattern of reasoning, which means that it is fallacious.
| |||