Conformism, imitativeness, submission to rules and to teachings is the writer's capital crime. The work of a writer must be not only the reflection, but the larger reflection of his personality. The only excuse that a man has for his writing is to write about himself, to reveal to others the sort of world that is mirrored in his own glass; his only excuse is to be original; he must speak of things not yet spoken of in a form not yet formulated...
(LH also gives the original French and a link to the post where he came across it.) One commenter asked how you can break the rules effectively if you have not mastered them first - a very common response to the sort of position Gourmont sets out, and one that misses the point.
Suppose I grow up in a family where people obsessively play Hearts. We switch around between different versions of the game - sometimes we play Black Maria, where the Queen of Spades costs you 13 points and you pass on three cards to the left before you begin play, sometimes we invent twists of our own. I also have four friends: A lives in a family of chess fanatics, B lives in a family of bridge fanatics, C lives in a family of go fanatics, D lives in a family of poker fanatics.
What I see at once is something remarkable. Languages are translatable, more or less; it may be more or less tricky, but it's intelligible to speak of Chinese being translated into Turkish AND Arabic AND English. Games are not translatable. Chess is a game for two players with complete information; you can't "explain" what's going on in a chess game in terms of bridge, which is a game for two sets of partners with imperfect information, a mixture of skill and chance which depends on skilful sharing of information between partners. And you can't "explain" either in terms of poker, which is a game for an indeterminate number of players, a mixture of skill and chance in which sharing of information between players would in fact be collusion and outlawed. A game is intelligible on its own terms - which means, paradoxically, that you can play a game with someone whose language you don't know, provided you both know the rules of the game.
You don't understand a game in terms of some other game, you understand it by learning to play it - but the more games you play, the more you will understand about the radical otherness of games. And this, it seems to me, is the sense in which Gourmont thinks each writer will develop his own aesthetics.
We don't have to agree with this position - it's perfectly possible to believe that a great writer may, on the contrary, be like a great chess player or a great bridge player, someone supremely gifted within the constraints of a particular established form. If originality is seen in terms of breaking rules, though, that presupposes that art is still comprehensible only in terms of constraints which already exist - originality is to be embedded in the sort of Oedipal drama at the heart of Bloom's Anxiety of Influence. It's as if one is actually incapable of understanding a form in which the rules one knows have no purchase. ("Yes, I realise it's a game for four people with cards, I know you don't have a board and you don't have pieces, but how can you expect to invent a game effectively if you haven't got the hang of the King's Indian?")