The rational is the reasonable, that which accords with reason. I conceive reason as an attitude whose principle is to shape thought, feeling, and action in accordance with objective values. Objective values are established by the average experience of external facts on the one hand, and of inner psychological facts on the other. Such experiences, however, could represent no objective ‘value’, if ‘valued’ as such by the subject; for this would already amount to an act of reason. But the reasoning attitude, which permits us to declare as valid objective values in general, is not the work of the individual subject, but the product of human history.
Most objective values—and reason itself among them—are firmly established complexes handed down to us through the ages, to the organization of which countless generations have labored with the same necessity with which the nature of the living organism, in general, reacts to the average and constantly recurring conditions of the environment, confronting them with corresponding function-complexes—as, for instance, the eye, which so perfectly corresponds with the nature of light. We might, therefore, speak of a pre-existing, metaphysical world-reason, if, as Schopenhauer has already pointed out, the reaction of the living organism that corresponds with average external influence were not the indispensable condition of its existence. Human reason, therefore, is merely the expression of human adaptability to the average occurrence which has gradually become deposited in solidly organized complexes, constituting our objective values. Thus the laws of reason are those laws which rule and designate the average ‘correct’ or adapted attitude. Everything is rational which harmonizes with these laws, and everything irrational (q.v.) which contravenes them.
Thinking and feeling are rational functions in so far as they are decisively influenced by the motive of reflection. They attain their fullest significance when in fullest possible accord with the laws of reason. The irrational functions, on the contrary, are such as aim at pure perception, e.g. intuition and sensation; because, as far as possible, they are forced to dispense with the rational (which presupposes the exclusion of everything that is outside reason) in order to be able to reach the most complete perception of the whole course of events.
As I make use of this term it does not denote something contrary to reason, but something outside the province of reason, whose essence, therefore, is not established by reason.
Elementary facts belong to this category, e.g. that the earth has a moon, that chlorine is an element, that the greatest density of water is found to be 4.0 centigrade. An accident is also irrational in spite of the fact that it may sustain a subsequent rational explanation.
The irrational is a factor of existence which may certainly be pushed back indefinitely by an increasingly elaborate and complicated rational explanation, but in so doing the explanation finally becomes so extravagant and overdone that it passes comprehension, thus reaching the limits of rational thought long before it can ever span the whole world with the laws of reason. A completely rational explanation of an actually existing object (not one that is merely postulated) is a Utopian ideal. Only an object that has been postulated can also be completely explained on rational grounds, since it has never contained anything beyond what was postulated by rational thinking. Empirical science also postulates rationally limited objects, since its deliberate exclusion of the accidental allows no consideration of the real object as a whole; hence empirical observation is always limited to that same portion of the object which has been selected for rational consideration. Thus, both thinking and feeling as directed functions are rational. When these functions are concerned not with a rationally determined choice of objects, or with the qualities and relations of objects, but with the incidental perceptions which the real object never lacks, they at once lose the quality of direction, and therewith something of their rational character, because they accept the accidental. They begin to be irrational. That thinking or feeling which is directed according to accidental perceptions, and is therefore irrational, is either intuitive or sensational. Both intuition and sensation are psychological functions which achieve their functional fulfillment in the absolute perception of occurrences in general. Hence, in accordance with their nature, their attitude must be set towards every possibility and what is absolutely accidental; they must, therefore, entirely forgo rational direction. Accordingly I term them irrational functions, in contrast to thinking and feeling, which reach perfection only when in complete accord with the laws of reason.
Although the irrational, as such, can never become the object of a science, nevertheless for a practical psychology it is of the greatest importance that the irrational factor should be correctly appraised. For practical psychology stirs up many problems that altogether elude the rational solution and can be settled only irrationally, i.e. they can be solved only in a way that has no correspondence with the laws of reason. An exclusive presumption or expectation that for every conflict there must also exist a possibility of rational adjustment may well prove an insurmountable obstacle to a real solution of an irrational character. (v. Rational)
Thinking and feeling are rational functions in so far as they are decisively influenced by the motive of reflection. They attain their fullest significance when in fullest possible accord with the laws of reason. The irrational functions, on the contrary, are such as aim at pure perception, e.g. intuition and sensation; because, as far as possible, they are forced to dispense with the rational (which presupposes the exclusion of everything that is outside reason) in order to be able to reach the most complete perception of the whole course of events. - CW 6, par.