SSH 105 Lecture Notes - Lecture 7: Modus Ponens, Modus Tollens

• School: Ryerson University
• Department: Social Sciences and Humanities
• Course: SSH 105
• Professor: David Hunter

Document Summary

Reasoning about alternatives and necessary and sufficient conditions. Often when we try to decide what to believe or do we are faced with several options, and we reason by ruling them out one by one. Either the maid did it or the butler did it; but the maid could not have done it; so, the butler is guilty. A disjunction asserts that at least one of the disjuncts is true. A disjunction is inclusive if all of its disjuncts can be true at once. It is a mistake to conclude that one disjunct is false just because the other one is true. Either steven is a student or a professor. Either jim is on the team or bob is on the team. If you know that a disjunction is true, and you discover that one disjunct is not true, then you can conclude that the other one must be true. This reasoning is a matter of ruling out possibilities.