Fall semester last year, a friend told me that arguments that either 1) aren’t true, or 2) have no reason to be believed, are not deductive arguments. This sounded odd to me, so let’s look at a few examples.
1) If it’s a poodle, then it’s a dog.
2) It’s a poddle
3) Therefore, it’s a dog
A standard modus ponens argument. True premises that lead to a true conclusion.
1) All dogs are cats
2) All cats are pancakes
3) Therefore, all dogs are pancakes
Every proposition in this argument is false, obviously
1) If God does not exist, then objective moral values do not exist
2) Objective moral values do exist
3) Therefore, God exists
Standard modus tollens.
So according to him, it would seem that the first argument is deductive because it’s true and there’s evidence to believe it. The second argument, however isn’t deductive because it’s obviously false. For the third argument, let’s pretend that there is no reason to believe it. Certain philosophers will give reasons to believe it, but let’s pretend there aren’t any. According to him, the argument wouldn’t be deductive because no reasons are given to believe it.
I’ve never heard anyone claim this before. All textbooks I’ve read consider those arguments deductive and any professor I ask at MU agree that they are. The deductive nature of an argument has to do with its structure, not with truth. So, perhaps he needs to take a logic class.