What are the two formal fallacies?
You asked: what are the two formal fallacies?
I wouldn't want to say that there are only two formal fallacies; are you are referring to the most common formal fallacies?
But perhaps this will help:
(I've written quite a bit in explanation. You might consider going straight to section 5 if you're in a rush.)
A fallacy, as you know, is a defect in an argument other than the falsity of the premises. (For example, the following argument does not contain a fallacy although it does not provide good reason for believing its conclusion.
All whales are mammals.
All mammals are blue.
Therefore, all whales are blue.
Notice that the argument is valid but the second premise is false.)
Fallacies can be divided into two groups: (1) formal and (2) informal fallacies.
A formal fallacy is basically a structural defect in an argument that is intended ...
This solution is comprised of an explanation of two basic rules of inference (modus ponens, and modus tollens) and their attendant fallacies. The concept of formal validity is also briefly discussed.