Rationality entails a kind of interchangeability. It's not about who said what, he said, she said, they said, X said it so it must be right, etc. Anyone can make the same argument, and the argument would still remain the same. It's about the argument and not the person. It doesn't matter who says it, and who says it does not affect the argument at all. The argument goes beyond the person, it goes outside of it, from the world of subjective to the world of objective.
Rationality is "removal of the subjective". Criticism of rationality is often always an emphasis of the subjective and to further away from objectivity. E.g. an appeal to emotion or emotional arguments. Or relativism that it depends on who's looking at it. That rationality is too cold, or that rationality is not everything and subjectivity must also be considered.
You'll notice that things like postmodernism or relativism is an argument for more subjectivity and removal of rationality and objectivity. No wonder then, that postmodernism turns into a nihilist thought that nothing is either right or wrong. This is because no amount of subjective argument can be persuaded to be either right or wrong. Subjective arguments are by definition right - to the person who's making the argument. This is due to the nature of subjectivity that it is self-referential and tautological. If you're right then you're right, if you're wrong then you're wrong. No one can persuade you otherwise and nothing can prove you to be wrong.
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So what does this have to do with Socionics? Socionics is by definition subjective, due to the way "typing" works. Typing is based on a subjective, personal experience of a person. But one can't convince others that one's own personal experience is correct or incorrect. It is simply not a rational, objective argument. One must instead make an argument that is rational and objective, by which that the argument can be made by anyone and still have the argument be left intact.
And that is what interchangeability is all about. And hence why, Socionics is an irrational system.
Of course, this is not limited to only Socionics. You'll notice that many things in this world, uttered by many people are irrational in the same way - that they tend to emphasis the subjective and attempt to remove the rational and the objective. Needless to say, those systems or thoughts often almost always fail or catastrophically crash in a tragic way. It's a thought of arbitrary dogmatism.