INTRODUCTION
Logic (Greek logos, “word,” “speech,” “reason”), science dealing with the principles of valid reasoning and argument. The study of logic is the effort to determine the conditions under which one is justified in passing from given statements, called premises, to a conclusion that is claimed to follow from them. Logical validity is a relationship between the premises and the conclusion such that if the premises are true then the conclusion is true.
The validity of an argument should be distinguished from the truth of the conclusion. If one or more of the premises is false, the conclusion of a valid argument may be false. For example, “All mammals are four-footed animals; all people are mammals; therefore, all people are four-footed animals” is a valid argument with a false conclusion. On the other hand, an invalid argument may by chance have a true conclusion. “Some animals are two-footed; all people are animals; therefore, all people are two-footed” happens to have a true conclusion, but the argument is not valid. Logical validity depends on the form of the argument, not on its content. If the argument were valid, some other term could be substituted for all occurrences of any one of those used and validity would not be affected. By substituting “four-footed” for “two-footed,” it can be seen that the premises could both be true and the conclusion false. Thus the argument is invalid, even though it has a true conclusion.
Logic (Greek logos, “word,” “speech,” “reason”), science dealing with the principles of valid reasoning and argument. The study of logic is the effort to determine the conditions under which one is justified in passing from given statements, called premises, to a conclusion that is claimed to follow from them. Logical validity is a relationship between the premises and the conclusion such that if the premises are true then the conclusion is true.
The validity of an argument should be distinguished from the truth of the conclusion. If one or more of the premises is false, the conclusion of a valid argument may be false. For example, “All mammals are four-footed animals; all people are mammals; therefore, all people are four-footed animals” is a valid argument with a false conclusion. On the other hand, an invalid argument may by chance have a true conclusion. “Some animals are two-footed; all people are animals; therefore, all people are two-footed” happens to have a true conclusion, but the argument is not valid. Logical validity depends on the form of the argument, not on its content. If the argument were valid, some other term could be substituted for all occurrences of any one of those used and validity would not be affected. By substituting “four-footed” for “two-footed,” it can be seen that the premises could both be true and the conclusion false. Thus the argument is invalid, even though it has a true conclusion.
