Rationality

To argue with a man who has renounced the use and authority of reason, and whose philosophy consists in holding humanity in contempt, is like administering medicine to the dead, or endeavoring to convert an atheist by scripture. ’Tis the last key-stone that makes up the arch…

Then stands it a triumphal mark! Then men

Observe the strength, the height, the why and when

It was erected; and still walking under,

Meet some new matter to look up, and wonder. Rationality is the use of logical reasoning in the areas of discourse and problem-solving. It is also the quality of being rational.

Definition
Like many other basic words, its meaning can be obscure and sometimes difficult. To be "rational" is generally considered to mean employing logical consistency and deriving appropriate conclusions from acceptable assumptions. Naturally, "consistency" and "appropriate" and "acceptable" all vary widely from person to person, which is one of the major problems with claiming to be "rational". Everyone will think they're being rational because they are happy with their assumptions and their own consistency. However, there are some accepted methods in place to try and standardise, such as in rationalism or science.

Typically, dictionaries define rationality as: the quality or state of being rational; the possession of reason; agreeableness to reason; reasonableness; the exercise of reason.

LessWrong
LessWrong’s take on the definition is as follows:
 * Rationality is thinking in ways which systematically arrive at truth.
 * Rationality is thinking in ways which cause you to systematically achieve your goals.
 * Rationality is trying to do better on purpose.
 * Rationality is reasoning well even in the face of massive uncertainty.
 * Rationality is making good decisions even when it’s hard.
 * Rationality is being self-aware, understanding how your own mind works, and applying this knowledge to thinking better.

Epistemic vs. instrumental rationality
Two types of rationality are often contrasted with each other: Epistemic and instrumental rationality. The former represents rationality in terms of determining and holding true beliefs. The latter consists of being able to make decisions that most achieve one's goals, whether or not said goals may be considered rational. This dichotomy is sometimes argued to be a false one by those who see instrumental rationality as a special case of epistemic rationality, or deny that intentions provide sufficient reason to take action toward their realization.

Bounded rationality
"Bounded rationality" is a term coined by Herbert Simon used to refer to a conception of agents opposed to "rational actor" models in which agents act with optimal or perfect rationality. Bounded rationality asserts that agents will — at least some of the time — not act optimally due to cognitive constraints. Instead, they do what Simon referred to as "satisficing", a portmanteau of "satisfy" and "suffice". That is, they will seek a solution that is "good enough" for their own purposes.

Ecological rationality
The concept of ecological rationality draws on bounded rationality and the strategy of satisficing. Ecological rationality posits that cognitive biases and heuristics are not inherently irrational but in certain conditions are preferable to a strict adherence to formal logic and probability theory. This is because the extra effort and time expended to consider all information in the framework of formal models of rationality does not give enough extra benefit over using a simple heuristic to make a decision. Thus, employing rough heuristics may be more "rational" than using what are normally considered "rational" methods.

Rationality vs. emotion?
Emotion is often contrasted with rationality. However, research in neuroscience and cognitive science, most notably by Antonio Damasio, has demonstrated that reason cannot function properly in the absence of emotion. Damasio investigated patients with damage to the ventromedial prefrontal cortex, which mediates activity between higher-order thought in the frontal lobe and emotional responses in the limbic system. Patients with this type of brain damage did not lose their ability to think logically, but suffered from a lack of emotional affect, rendering them unable to make ordinary, everyday decisions. Without emotional affect to (among other things) assign values to what are often arbitrary choices, the patients could generate and weigh different alternatives against each other in their minds, but were incapable of actually deciding between them.

This might be conceived of as a case similar to the problem of Buridan's ass. In this thought experiment, an ass is placed in a position equidistant from two stacks of hay. The ass will move toward the closest stack of hay, but remains stuck in its initial position because neither one is closer and so will starve itself because of its inability to choose "rationally" between the stacks. If the ass picks one stack at random to avoid starving, is this decision rational? This question has invited much navel-gazing over the definition of rationality in philosophical arenas.

Rationality and discovery
Critical Rationalism can be seen as complementary to serendipity in scientific discovery. Yet cognitive science made rationality a subject even in the area of finding new theories, and laws, started by the computational models in many machine learning areas. These movements focus on any rational process, and see scientific discovery as the main challenge of any creative rational learning process, in science, and in the long run, in general, by modelling the whole brain logically, on a computer.

Rationality and power
A number of thinkers have investigated the notion of rationality, or what is considered "rational", as being intertwined with Max Weber and Michel Foucault, for example, focused on the use of instrumental rationality in the process of "rationalization," i.e., how bureaucratic power imposes "rational" methods on citizens for its own ends.

Relation to rationalism
Rationality should not be confused with rationalism. Rationality is used to describe a quality of discourse or behavior; rationalism is a philosophy recognizing reason as the prime (or only) "way of knowing".

Alternatives
There are, of course other ways of knowing, but these have the disadvantage of not working.