Art is a social function. This is not a Marxist demand, but arises from the very way in which art forms are defined. Only those things are recognised as art forms which have a conscious social function. The phantasies of a dreamer are not art. They only become art when they are given music, forms or words, when they are clothed in socially recognised symbols, and of course in the process there is a modification. The phantasies are modified by the social dress; the language as a whole acquires new associations and context. No chance sounds constitute music, but sounds selected from a socially recognised scale and played on socially developed instruments.

It is not for Marxism therefore to demand that art play a social function or to attack the conception of ‘art for art’s sake’, for art only is art, and recognisable as such, in so far as it plays a social function. What is of importance to art, Marxism and society is the question: What social function is art playing? This in turn depends on the type of society in which it is secreted. . . .
But what is art as a social process? What is art, not as a mere art work or a means of earning a living, but in itself, the Part it plays in society? I have dealt fully with this point elsewhere, and need only briefly recapitulate now.

The personal phantasy or day-dream is not art, however beautiful. Nor is the beautiful sunset. Both. are only the raw material of art. It is the property of art that it makes mimic pictures of reality which we accept as illusory. We do not suppose the events of a novel really happen, that a landscape shown on a painting can be walked upon – yet it has a measure of reality.

https://www.marxists.org/archive/caudwell/1938/studies/ch03.htm