modus ponens and modus tollens

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(Latin: "method of affirming" and "method of denying") In logic, two types of inference that can be drawn using a hypothetical proposition i. e., from a proposition of the form "If p, then q" (symbolically p q). Modus ponens refers to inferences of the form p q; p, therefore q. Modus tollens refers to inferences of the form p q; q, therefore, p. An example of modus tollens is the following: "If an angle is inscribed in a semicircle, then it is a right angle; this angle is not a right angle; therefore, this angle is not inscribed in a semicircle