Much everyday reasoning runs on 'if... then' statements, and four patterns account for most of it. Two are valid. Modus ponens: if it rained, the street is wet; it rained; so the street is wet. Modus tollens: if it rained, the street is wet; the street is not wet; so it did not rain. Two look almost identical but are invalid. Affirming the consequent: if it rained, the street is wet; the street is wet; so it rained. But a burst pipe or a street cleaner could have wet it. Denying the antecedent: if it rained, the street is wet; it did not rain; so the street is not wet. Same problem. The key is that 'if X then Y' says X is enough for Y, not that X is the only way to get Y. The invalid patterns are common in diagnosis, blame and marketing: 'successful people wake early; I wake early; so I will succeed' is affirming the consequent. When you meet an if-then argument, name which of the four patterns it follows, and you know at once whether the structure holds. After this Concept you can classify an if-then argument as one of the four patterns and say whether its structure is valid.
Modus ponens and modus tollens are two basic rules of logic that tell you when an "if-then" argument is valid. Modus ponens says: if P then Q, and P, so Q; modus tollens says: if P then Q, and not Q,…