Proust wants to emphasise the distance between the writer and the object that he pursues in his writing. As he continues in this letter:
It has to do with drawing a reality out of the unconscious in such a way as to make it enter into the realm of the intellect, while trying to preserve its life, not to garble it, to subject it to the least possible shrinkage -- a reality which the light of intellect alone would be enough to destroy, so it seems. To succeed in this work of salvage, all the forces of the mind and even of the body, are not superfluous. It is a little like the cautious, docile, intrepid effort necessary to someone who, while still asleep, would like to explore his sleep with his mind without this intervention leading to his awakening. Here precautions must be taken. But although it apparently embodies a contradiction, this form of work is not impossible.
Unfortunately I don't have the original French for this letter. The English is from the 1950 translation by Mina Curtiss. In Ronald Hayman's biography of Proust, the translation he cites (which might be his own) is:
It is a matter of drawing something out of the unconscious to make it enter the domain of consciousness, while trying to preserve its life, [not to] mutilate it, to keep leakage to a minimum -- a reality which could apparently be destroyed by exposure to the light of mere intelligence. To succeed in this work of salvage, the whole strength of the body and the mind is not too much. Something like the same kind of effort -- careful, gentle, daring -- is necessary to someone who while still asleep would like to examine his sleep with his intelligence, without letting this interference wake him up.
I like the way, in Hayman's translation, his use of the words 'mutilate' and 'leakage' evoke the delicate, membranous anatomy of some unknown submarine creature; Curtiss's 'shrinkage' and 'garble' deaden the image, turning it into something like a cross between an expensive shirt and a telephone message.