Definitions for Key Concepts in Logic

Logic = The branch of philosophy that studies inference and argument—i.e., the area that studies the proper movement of mind from one idea/belief to another for the purpose of avoiding inconsistencies in one’s thinking.

Argument = a set of claims, one of which (the conclusion) is supposed to be supported by the other(s) (the premise(s))

Inference = the support that the premises of an argument provide for the conclusion of the argument

Deductive argument = an argument such that, on the assumption that the premises are true, the conclusion would necessarily be true as well.

Inductive argument = an argument such that, on the assumption that the premises are true, the conclusion would be true to some degree of probability, but would not necessarily be true

Valid argument = an argument is valid when the truth of its premises guarantee the truth of its conclusion—i.e., if the premises were true, then the conclusion would have to be true as well. Also sometimes called a “deductively valid” argument.

Invalid argument = an argument that is not valid—i.e., even if the premises were true the conclusion could still be false.

A cogent argument = an argument such that, on the assumption that the premises are true, the conclusion would probably be true, but not necessarily.

A non-cogent argument = an argument such that even if the premises were true, the conclusion would not even be likely to be true—i.e., the premises can be true and the conclusion still unlikely.

A sound argument = an argument is sound if it has a valid inference and all its premises are true.

An unsound argument = an argument that is not sound

A strong argument = an argument is strong if it has a cogent inference and all its premises are true.

A weak argument = an argument that is not strong.

A good argument = an argument is good if it gives a solid basis for believing its conclusion. The two kinds of good arguments are those...