Modus Tollens - Explanation

Explanation

The argument has two premises. The first premise is the conditional "if-then" statement, namely that P implies Q. The second premise is that Q is false. From these two premises, it can be logically concluded that P must be false.

Consider an example:

If the watch-dog detects an intruder, the dog will bark.

The dog did not bark

Therefore, no intruder was detected by the watch-dog.

Supposing that the premises are both true (the dog will bark if it detects an intruder, and does indeed not bark), it follows then that no intruder has been detected. This is a valid argument since it is not possible for the premises to be true and the conclusion false. (It is conceivable that there may have been an intruder that the dog did not detect, but that does not invalidate the argument; the first premise goes "if the watch-dog detects an intruder." The thing of importance is that the dog detects or doesn't detect an intruder, not if there is one.)

Another example:

If I am the axe murderer, then I used an axe.

I cannot use an axe.

Therefore, I am not the axe murderer.