In philosophy, the word "rationality" has been used to describe numerous religious and philosophical theories, especially those concerned with truth, reason, and knowledge. Rationality of various kinds has often been taken to be one of the key features of the mind and/or soul that separates humans from animals. Continental Rationalism is a school in which rationality and reason are the key methods by which we obtain knowledge, in opposition to empiricism which states that knowledge is obtained primarily via the senses.

A logical argument is sometimes described as rational if it is logically valid. However, rationality is a much broader term than logic, as it includes "uncertain but sensible" arguments based on probability, expectation, personal experience and the like, whereas logic deals principally with provable facts and demonstrably valid relations between them. For example, ad hominem arguments are logically unsound, but in many cases they may be rational.

In economics, sociology, and political science, a decision or situation is often called rational if it is in some sense optimal, and individuals or organizations are often called rational if they tend to act somehow optimally in pursuit of their goals. Thus one speaks, for example, of a rational allocation of resources, or of a rational corporate strategy. Sometimes, in this context, rationality is equated with behavior that is self-interested to the point of being selfish. Sometimes rationality implies having complete knowledge about all the details of a given situation. See Rational choice theory.

Debates arise in these three fields about whether or not people or organizations are "really" rational, as well as whether it make sense to model them as such in formal models. Some have argued that a kind of bounded rationality makes more sense for such models. Others think that any kind of rationality along the lines of rational choice theory is a useless concept for understanding human behavior; the term Homo economicus is largely in honor of this view.

Rationality is a central principle in Artificial Intelligence, where an rational agent is specifically defined as an agent which always chooses the action which maximises its expected performance, given all of the knowledge it currently possesses.

In a number of kinds of speech, "rational" may also denote a hodge-podge of generally positive attributes, including:

• reasonable: "having sound judgement" (Webster's)
• reasonable: "not extreme or excessive" (Webster's)
• justifiable on the basis of reason (as opposed to tradition or emotional response).
• not foolish
• sane
• good

Perhaps useful contrasts (??):

• rational vs. emotional
• rational vs. creative
• rational vs. spiritual
• reason vs. divine truth

See also

• Rational number