Practical Guide to Clear Thought Ep.13

How do we evaluate a deductive argument?
For a deductive argument, the criteria are truth, validity and soundness.

What is truth?
A statement is true when its claim corresponds to reality. For example, the statement "snow is white" is true because in reality snow is indeed white. However, the statement "the moon is made of green cheese" is false because in reality the moon is not made of green cheese. Note that truth and falsity apply only to statements (as opposed to arguments).

What is validity?
A deductive argument is valid when its premisses entail (lead to) its conclusion, that is, when it is not possible for its premisses to be true and its conclusion false. For example: "All heavenly bodies made of green cheese are bright. The moon is a heavenly body made of green cheese. Therefore the moon is bright." Two things must be noted here. First, this criterion does not require the premisses to be true; it is enough that they would lead to the conclusion if they were true. Second, validity applies only to arguments, as opposed to statements. (Note that the word for a handicapped person is also spelt "invalid". The pronunciation, however, is different. The word for a handicapped person is pronounced "IN-ver-lid", while the word for a deductive argument where the premisses do not lead to the conclusion is pronounced "in-VAIR-lid".)

Does this mean we cannot say a statement is valid?
The word "valid" is commonly used as meaning "true" (as in "it's valid to say the moon is a heavenly body"), but this is technically wrong. "True" and "false" apply to statements. "Valid" and "invalid" apply to arguments.

Next: What is soundness?

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