ABSTRACT
The first part of this book is about logic in general, rather than any one system. I shall end it by discussing a way of arguing that is both indispensable to life and, in its commonest forms, fallacious. This is argument by analogy. Earlier I invented the category of USELESS VALIDITY: arguments with contradictory premises for example. Their form will not allow true premises and a false conclusion, but they can't prove anything. Argument by analogy is often a USEFUL FALLACY. Like many fallacies, it may take alternate forms: either invalid, or valid with an unacceptable premise.
