This is a good one. Anybody know the source off-hand?

The quotation is from Appreciations and Criticisms by G.K Chesterton, Pickwick Papers,
CHAPTER III

However this may be in the matter of religion and philosophy, it can be said with little exaggeration that this truth is the very key of literature. The whole difference between construction and creation is exactly this: that a thing constructed can only be loved after it is constructed; but a thing created is loved before it exists, as the mother can love the unborn child. In creative art the essence of a book exists before the book or before even the details or main features of the book; the author enjoys it and lives in it with a kind of prophetic rapture. He wishes to write a comic story before he has thought of a single comic incident. He desires to write a sad story before he has thought of anything sad. He knows the atmosphere before he knows anything. There is a low priggish maxim sometimes uttered by men so frivolous as to take humour seriously -- a maxim that a man should not laugh at his own jokes. But the great artist not only laughs at his own jokes; he laughs at his own jokes before he has made them. In the case of a man really humorous we can see humour in his eye before he has thought of any amusing words at all. So the creative writer laughs at his comedy before he creates it, and he has tears for his tragedy before he knows what it is. When the symbols and the fulfilling facts do come to him, they come generally in a manner very fragmentary and inverted, mostly in irrational glimpses of crisis or consummation. The last page comes before the first; before his romance has begun, he knows that it has ended well. He sees the wedding before the wooing; he sees the death before the duel. But most of all he sees the colour and character of the whole story prior to any possible events in it. This is the real argument for art and style, only that the artists and the stylists have not the sense to use it. In one very real sense style is far more important than either character or narrative. For a man knows what style of book he wants to write when he knows nothing else about it.

Pickwick is in Dickens's career the mere mass of light before the creation of sun or moon. It is the splendid, shapeless substance of which all his stars were ultimately made...

At its foundations, all constructions (except "The Factor Cheval" or some kind of art) must have always been an architectural plan.
A construction it´s a creation, a human creation too and it can be loved before exists in a material form (architect´s brain).
Creations are ideas or dreams made real.