So what does it mean to be rational? We usually distinguish between two kinds of rationality. Epistemic rationality, which is involved in acquiring true beliefs about the world and which sets the standard for what we ought to believe, and instrumental rationality which is involved in decision-making and behavior and is the standard for how we ought to act.
We are epistemically rational if we believe things for which we have good evidence and if we would change our beliefs in light of evidence against those beliefs. For example, it is epistemically rational for me to believe there is some wine in the fridge since it was left over from last night and I put it there myself. But that is not enough; I should also be willing to change my belief, for example if I am told that someone drank it in the meantime and maybe if I saw for myself that the wine in the fridge is no more. To still believe, for no other reason, that there is wine in the fridge despite this new evidence would be epistemically irrational.
We are instrumentally rational when we act in ways that are appropriate for achieving our goals – for instance when we go on a diet when we want to lose weight, or we study hard to do well in an exam.
In contrast, one is irrational when one’s beliefs or actions are not in accord with the requirements of rationality. For instance, if one wants to achieve a certain goal but acts in ways that do not lead to that goal; when one forms beliefs for which there is no evidence, or that fly in the face of available evidence; when one reasons faultily and so on. And, because we value rationality and hold it up as a standard we should aspire to, irrationality is understood to be a bad thing.Useful article. The above excerpt is the takeaway.
Recently, philosopher and head of the PERFECT project Lisa Bortolotti has offered evidence that though we tend to rationalize our decisions after we have made them, we do not make choices primarily by rational deliberation. Instead, most of our decision-making involves emotions and intuition and, often, these processes lead to better results than those achieved by reasoning through our choices.
BTW, there are five criteria of truth. 1) The first is correspondence or conformity with facts and evidence. This is also known as epistemological or semantic truth. 2) The second is coherence or consistency, which is logical validity and mathematical consistency. This is also known as syntactical truth. 3) The third criterion is pragmatic or instrumental. It is suitability to an objective. It is also known as practical truth or practical wisdom. 4) The fourth criterion is simplicity or elegance. It is the most economical expression. 5) The fifth criterion is the coincidence of the previous four. It is called completeness or comprehensiveness.
"Rationality" is considered the ability to apply all five consistently without the intrusion of subjectivity, that is, emotion, preference, and cognitive-affective bias. Current research suggests that rationality in this sense is an unrealizable ideal owing to human brain functioning as revealed by cognitive science. The rational and non-rational are entangled in brain functioning to the degree that they cannot be completely unentangled. Moreover, the entanglement is subliminal and humans are not even aware of its operation. "You" as the conscious and intentional mind are the slave of your brain.
The Irrationality Within Us
Elly Vintiadis, philosophy instructor at the American College of Greece
ht Mark Thoma at Economist's View