Logic is the study of reasoning, proof, thinking, or inference. Logic enables us to determine whether a given piece of reasoning is valid or invalid. One doesn't need to study logic in order to reason correctly. Most of us already use the rules of logic when making decisions in our lives, even though we may not know it. However, knowledge of logic can improve our ability to make decisions, help us argue for our beliefs logically, and help us analyze the arguments of others.

The basic building block of a logical argument is a *proposition*.
A proposition is a statement that is either true or false, but not both.
For example, the following statements are propositions:

- All men are mortal.
- This book is difficult to read and expensive.
- Some cows are blue.

The following statements are not propositions, since we cannot say that they are either true or false:

- Come here right now!
- Is it raining outside?
- This statement is false. (See the paradoxes page for more about this statement).

An *argument* consists of a series of propositions connected in
a manner to establish some other proposition. An argument consists of
three steps: First, the propositions necessary for the argument (called
premises) are stated. Next, the premises are used to infer other propositions.
Finally, the proposition that was originally set out to be established
is affirmed on the bases of the premises and inferences. This is the
conclusion of the argument. Here is a simple example:

- All men are mortal. (premise)
- Socrates is a man. (premise)
- Therefore Socrates is mortal. (inference and conclusion)

Traditionally, there are two types of arguments: A *deductive
argument* provides conclusive proof of its conclusions.
In an *inductive argument*, the premises provide
evidence for the conclusion, but do not conclusively establish it.
Deductive arguments can be either valid or invalid. A valid argument
is one that is well-formed. If the premises of a valid argument
are all true, then the conclusion will be true. However, if the
premises are false, then the conclusion may or may not be true.

To delve further into logic, it can be helpful to investigate propositional logic.

A fallacy is used to refer to an argument that is technically incorrect, even though it may seem to be valid. There are two types of fallacies: formal and informal. Formal fallacies involve errors in the form or structure of the argument, while informal fallacies involve other errors.

See also recreational logic.