Genius is an aptitude to produce something for which no definite rule can be postulated; it is not a capacity or skill for something that can be learnt from some rule or other. Its prime quality, then, must be originality . . . . The aptitude cannot of itself describe how it creates its products, or demonstrate the process theoretically, though it provides the rules by itself being a part of nature. Thus the progenitor of a work of art is indebted to his own genius and he does not himself know how the ideas for it came to him, not does it lie within his power to calculate them methodically or, should he so wish, to communicate them to others by means of principles that would enable others to create works of equal quality. It is through genius that nature prescribes the rules of art.This is from Immanuel Kant's Kritik der Urteilskraft, published in Berlin in 1790. I found this passage, translated by Peter le Huray and James Day, quoted in Unfinished Music by Richard Kramer.
Here is the full passage in a translation by J.H. Bernard
Beautiful Art is the art of geniusYou can read all of Berhard's 1914 translation of the Critique of Judgement on line!
Genius is the talent (or natural gift) which gives the rule to Art. Since talent, as the innate productive faculty of the artist, belongs itself to Nature, we may express the matter thus: Genius is the innate mental disposition (ingenium) through which Nature gives the rule to Art.
Whatever may be thought of this definition, whether it is merely arbitrary or whether it is adequate to the concept that we are accustomed to combine with the word genius (which is to be examined in the following paragraphs), we can prove already beforehand that according to the signification of the word here adopted, beautiful arts must necessarily be considered as arts of genius.
For every art presupposes rules by means of which in the first instance a product, if it is to be called artistic, is represented as possible. But the concept of beautiful art does not permit the judgement upon the beauty of a product to be derived from any rule, which has a concept as its determining ground, and therefore has at its basis a concept of the way in which the product is possible. Therefore, beautiful art cannot itself devise the rule according to which it can bring about its product. But since at the same time a product can never be called Art without some precedent rule, Nature in the subject must (by the harmony of its faculties) give the rule to Art; i.e. beautiful Art is only possible as a product of Genius.
We thus see (1) that genius is a talent for producing that for which no definite rule can be given; it is not a mere aptitude for what can be learnt by a rule. Hence originality must be its first property. (2) But since it also can produce original nonsense, its products must be models, i.e. exemplary; and they consequently ought not to spring from imitation, but must serve as a standard or rule of judgement for others. (3) It cannot describe or indicate scientifically how it brings about its products, but it gives the rule just as nature does. Hence the author of a product for which he is indebted to his genius does not himself know how he has come by his Ideas; and he has not the power to devise the like at pleasure or in accordance with a plan, and to communicate it to others in precepts that will enable them to produce similar products. (Hence it is probable that the word genius is derived from genius, that peculiar guiding and guardian spirit given to a man at his birth, from whose suggestion these original Ideas proceed.) (4) Nature by the medium of genius does not prescribe rules to Science, but to Art; and to it only in so far as it is to be beautiful Art.