PHIL 2410 Lecture Notes - Lecture 2: Propositional Calculus, Barack Obama, Michelle Obama
28 Sep 2017
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An argument is logically valid if and only if it is not possible for the premises all to be true and the conclusion to be false. To disprove this definition you need an example: all cats are trees. All cats are mammals: they work together to create truth through falsehood, this alternate definition does not seem to work. I said last time that logicians, as logicians, cannot tell us when a se(cid:374)te(cid:374)(cid:272)e is true or false. It(cid:859)s the (cid:271)usi(cid:374)ess of all other s(cid:272)ie(cid:374)(cid:272)es (cid:862)the grass is gree(cid:374)(cid:863). I also said that there was an exception to this. In a very small set of cases, logic can tell us that a sentence is true or false. This occurs when the sentence is logically true or logically false. You do not have to check to see if its true. We do not need to consult experts or even the world to determine that this sentence is true.
