Medical Ethics Darin Harootunian 9/12/11 2 We can identify deductively valid arguments simply by their form (at least in almost all cases). W e don’t need to consider the truth of the premises to determine whether the argument is valid or invalid. Here are some valid inference forms that we’ll be using this semester: Modus Ponens 1) If P, then Q. 2) P Thus, 3) Q. Modus Tollens 1) If P, then Q. 2) ~Q. Thus, 3) ~P. I’ll give you some o ther valid inference when we come across them in the course – there are many . For now , let’s consider two invalid arguments: Argument G 1) If John is a cardiologist, then John is a medical doctor. 2) John is not a cardiologist. Thus, 3) John is not a medical doctor. Argument H 1) If John is a cardiologist, then John is a medical doctor. 2) John is medical doctor. Thus, 3) John is a cardiologist. Why are these arguments invalid? Think about it. For an invalid argument, there is always a counterexample to it. A counterexample is a scenario in which the premises of the
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