As a general rule, a photo should have a clearly defined subject. However, in some cases, the subject is left deliberately indiscernible. Is abstract a valid form of photography? Is it art, or is it a mindless melding of color?
What is the purpose of it?
When you create a portrait, there is a preservation of the memory of a person. Likewise, a landscape shot preserves the beauty of a landscape. Weddings, events, sports/action, wildlife; these all have practical functions. Abstract is very different. What point is made? What emotion is conveyed? What property does the indefinable possess that could qualify it as art? Abstract photography is aesthetically pleasing, but that often seems to be the scope of it.
Does it actually involve composition?
That is one thing you can be certain of. Lines, curves, sharpness, blur, and color all play into the image. And of course, things like the rule of thirds and filling the frame can still be used. The reason being that while the subject may very well be one that cannot be readily identified, the goal is still to create visual appeal. In reality, the composition IS the image. In other modes of photography composition enhances the image, but in abstract form is everything.
So back to the question…
Can art be art when there is form but seemingly no conveyance? Does photography exist when there is no clear subject? Is beauty alone substance enough? Those are questions that must be answered by the photographer.