A book is nothing but a cube of hot, smoking conscience.

Some of the letters between Boris Pasternak and Marina Tsvetayeva reference Pasternak's "A Few Principles," which had been published some years earlier.

Letters: Summer 1926 summarizes and quotes some of these principles to provide background information.

2. Contemporary trends assume that art is like a fountain, when really it is like a sponge.

They have decided that art ought to gush, but it ought, rather, to suck up and absorb.

They assert that art can be divided into categories according to means of representation, when actually it is composed of organs of perception.

Art must always remain among the spectators and see things more clearly, more truthfully, more perceptively than the others, but in our day it has resorted to using face powder and dressing rooms and displaying itself on the stage. It is as if there were two forms of art and one of them, knowing that it holds the other in reserve, allows itself the luxury of perversion, which is tantamount to suicide. It makes a display of itself when it ought to get lost in the top gallery, in anonymity, and be unaware that it cannot help being discovered, that while shrinking in the corner it is afflicted with a glowing translucence, the phosphorescence that goes with certain diseases.

3. A book is nothing but a cube of hot, smoking conscience.

[…] One forgets that the only thing within our power is the ability to keep the voice of truth within us undistorted.

The inability to find and speak the truth is a failing that no talent for speaking the untruth can disguise.

Perhaps art is a fun-house mirror.

You can find some thoughts on these principles at Brain Pickings.