PHILOS 2CT3 Chapter Notes - Chapter 4: Modus Ponens, Modus Tollens, Disjunctive Syllogism
5 Mar 2018
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Like modus ponens, it begins with a conditional: but, rather than affirming the antecedent in the second premise, it denies the consequent. By denying the consequent of the first premise, it can be deduced that the antecedent is not true. Ex: p1 = if pictures are hanged, then people are hung, p2 = it is not the case that people are hung, c = it is not the case that pictures are hanged. Combines two conditional claims from the premises to create a new conditional claim. Commonly used in the process of elimination. Ex: a detective might know that either sue or jack committed the murder: discovering evidence that exonerates sue from having committed the murder, the detective concludes that jack committed the murder. Given that it must be either p or q from the first premise, the second premise denies that p is true, leaving only q as true. Valid forms commonly found in public debate.
