What is Logic?

Logic is the study of the methods and principles used to distinguish correct reasoning from incorrect reasoning. Author of several books on logic and mathematics, Irving M. Copi wrote, "The distinction between correct and incorrect reasoning is the central problem with which logic deals."

Put more simply, logic is the science of reasoning.

What is reasoning? Reasoning can be defined as a type of thinking in which an inference occurs.

What is an inference? An inference is the process of drawing a conclusion from a one or more propositions which are either true or false.

What is a proposition? A proposition is merely a sentence that is either true or false.

Propositions are grouped to form arguments. An argument is a group of propositions, one leading to the next, that results in a conclusion.

Arguments, then, are structured forms of reasoning.

Arguments can be deductive or inductive. In a deductive argument, the conclusion necessarily follows from the propositions. For example:

All humans are mortal.
Socrates is a human.
Therefore, Socrates is mortal.

Deductive arguments are evaluted as either valid or invalid. This is important to understand because deductive argument can be valid but untrue. For example:

All humans have 3 legs.
Roy is a human.
Therefore, Roy has 3 legs.

The argument above has a valid form but its untrue premises render it invalid.

An inductive argument proceeds from specific observations to a general conclusion. For example:

All cows are mammals and have lungs.
All dogs are mammals and have lungs.
All cats are mammals and have lungs.
Therefore, all mammals (probably) have lungs.

Unlike the deductive argument, in an inductive argument, the propositions provide support for the conclusion, but the conclusion doesn't necessarily follow. Inductive arguments are evaluted as better or worse based on the strength of their propositions.

So, in conjunction with the Laws of Thought, the task of the logician is to:
1. Identify the form of the argument
2. Determine the validity of the deductive argument
3. Evaluate the strength of an inductive argument.

References:
Irving M. Copi, Introduction to Logic, 7th ed. (New York: Macmillan, 1986)
Geisler, Norman L., and Frank Turek, I Don't Have Enough Faith to be an Atheist, Crossway Books, 1300 Crescent St., Wheaton IL 60187, ©2004.