A fact is something that can be demonstrated to be true, or verified. That a certain number of births were recorded in the United States last year, that it was raining on a particular night at Kennedy Airport, that water boils at 212 degrees Fahrenheit—all these are facts that can be verified beyond a reasonable doubt. Obviously, the appropriate facts provide the strongest possible support for an argument.
Two cautions are to be observed in using facts as evidence. First, the facts must be accurate. Unfortunately, there can be dishonesty in argument, and the writer who does not check his (sic) facts carefully is dishonest. Second, the facts must be plentiful enough to be convincing. How many facts it takes to convince your reader will depend on the nature of the subject and the impressiveness of the facts themselves, but, in general, the more facts you have the stronger your argument will be.
Chapter 23 Language and Logic, Evidence in Warriner's English Grammar and Composition Revised Edition, Complete Course, John E. Warriner and Francis Griffith, 1965