Sartre, in the opening chapter of his very challenging read, Being and Nothingness, cleaves existence neatly in two; what he calls being-for-itself and being-in-itself.
The in-itself is being. I don’t recall Sartre ever explicitly describing it as physical matter, but that is basically what it amounts to. The in-itself is characterised by three features: 1) it is in-itself, 2) it is what it is, and 3) it is. Respectively, these mean: 1) the in-itself is independent; i.e. it doesn’t depend on anything else to exist, 2) it doesn’t refer to itself; i.e. it isn’t self-reflexive, and 3) it is neither possible nor necessary. It isn’t necessary because it didn’t have to be, but neither is it possible because inert, non-conscious matter has no possibilities.
The for-itself, on the other hand, is consciousness. What does this mean? Consciousness is precisely not being. It is an empty, ‘massless’ perspective on, or relation to, being. The for-itself cannot be grasped because it is not a being, it’s not a thing, it is precisely no-thing… which is not the same as saying it is an illusion or that it doesn’t exist at all. If you find this scientifically implausible, I challenge you to describe consciousness in a way that preserves what conscious clearly is, all while staying within the confines of naturalistic materialism.