Maybe in an extremely technical sense it is a difficult question, but the simple semantic meaning of irrationality conforms to that simple formula — introbert

Sorry for the double post. Irrationality, what is it?

A variety of things. It can be considered a type of passion such as being persistent about an argument. It can be considered the opposite of reason. It can be a way of acquiring knowledge, such as by intuition, imagination or divination. It can be a type of position such as nihilist, solipsism, or sophistry. And more but the work-bell has just rung.

Edit: as in sophistry, using fallacies such as false appeals etc. but even these can be powerful. Taking an extreme position is also irrational which is ironic in the case of extreme rationality such as scientism. Naturally a moral position is irrational if one argues in the virtues of immorality or amorality. Anything analogous to mental disorder such as postmodernism=schizophrenia or using paranoia to delve into the hidden motivations, inspirations, intentions and consequences etc. of a particular position. Having a fixed idea is also an irrational problem in philosophy but this kind of obsession can result in getting lots of mental work done. My own fixation on irony is about something irrational because it is about a discrepancy between idea and reality. Work-bell again.

There are also irrational states such as being angry, or under the influence of drugs such as mescaline which some claim to have transcendental effects. Some forms of transcendence especially when the thing being transcended is the basis for rationality like experience in empiricism or the moral order in transvaluation. Thinking about philosophy idly is an irrational practice compared to methodical, disciplined work. There is much more actually but lunch is almost over.

So to attempt a definition of irrationality in isolation is difficult, but as the opposite or absence or negation of rationality is easiest. Rationality is not just one thing like logic, it is a number of things like objectivity, empiricism, following rules, being of sound mind, making sense, organized, following a method and more.

So to attempt a definition of irrationality in isolation is difficult, but as the opposite or absence or negation of rationality is easiest — introbert

As the opposite/absence/negation of rationality, ok. What is rationality then?

Irrationality represents one abstract ideal but it is so many varied real things as logical fallacies, to intuition, to emotion, to divination, to imagination and so much more. — introbert

The antithesis of philosophy (i.e. reflective / dialectical discursive practices aka "reason").

I'll attempt two definitions one "irrational" and another "rational".

Irrational: Rationality is conformity in thought and action to socially constructed regimes of truth that determine what is done and said as well as forms the basis for evaluating in a moral sense what is deemed true or false, right or wrong and good or bad.

Rational: Rationality is the objective, logical and empirical basis of thought and action that establishes norms for forming consensus on what is considered true or false, right or wrong and good and bad.

I sympathise (emotionally) with your position about rationality, and irrationality as the antithesis of philosophy. However, irrationality is too vast a field to be banished by broad strokes from philosophy. There is much to say on the topic from classical greek philosophy to Kierkegaard and Nietzsche, but I will not appeal to irrationalism's role in the history of philosophical thought. My own fixation on irony which is reducible to discrepancy between idea and reality is irrational but it connects to postmodernism and the irrational definition that precedes this paragraph. A definition of rationalism that is relativistic or contingent is opposed to the expectations of objectivity and realism that would be found in a contemporary definition. To deny irrationality in philosophy would be to deny descent in the face of sanctions used to preserve the established rules. If objectivity is deindividuating, a subjective individual position appears irrational to a social group that has established norms based on this value.

Rationality can get you killed (vide Socrates, death by hemlock).

Socrates used logic but was not rational in the sense that he was often challenging the basic ideas of Athenian society. It is fundamentally irrational to do something that can get you killed.

Socrates used logic but was not rational in the sense that he was often challenging the basic ideas of Athenian society. It is fundamentally irrational to do something that can get you killed. — introbert

Right you are amigo! Socrates was irrational but logical, si? Care to expand on the difference between the two?

It's possibly complicated, but I look at it as something fairly simple. At bottom I think people are basically irrational. With effort we can train our minds to think with skill, using logic and empirical methods etc. I see being dichotomously dual natured, irrational-rational, as being more plausible than being completely one or the other. There are other such things as individual-collective: no man is an island but everyone is an individual. So irrational-logical features this dichotomy, which is embodied in the ironic nature of Socrates' method of feigning ignorance, but then using logic and questioning. I conclude feigning ignorance is an irrational method for the simple reason that it uses deception. To lie is not a rational practice in most situations. That someone can behave irrationally, but think logically, is not nearly unprecedented. It is a regular occurrence to argue with anger, which is an irrational state, in a logical way. Just because two things are opposing doesn't mean they cant exist at the same time, or are unstable together.