I believe that the subject, or the mind, is one element of every perceived experience, in other words, that reality invariably has a subjective pole, aspect or element, which is not generally apparent; as Michel Bitbol says 'it knows but it is not known'. This is where I differ from realism which attributes to the objects of perception intrinsic reality presumably anchored by physics.
A Buddhist would say that 'subject and object' or 'mind and world' are co-arising; that these exist in dependence on one another, so, no world without mind, but conversely no mind without world (which is where Buddhism departs radically from European idealism). Whereas the realist invariably recoils at this notion, because we know that we are finite, temporal beings, while the world is of vast duration and size that extends far beyond us in both space and time. But again, I would counter that the notions of 'duration' and 'scale' both imply or require a mind, as they too have no intrinsic reality. I mean, even though it is empirically true that the world is much vaster than myself, the sense of scale which enables us to judge what is larger or smaller, sooner or later, nearer or further, seems to me to be not findable in the physical world. That is dependent upon a perspective.
Mystical, I suppose, but then Neils Bohr did say 'a scientist is just an atom's way of looking at itself'.
But we have to consider that the human mind in a biological sense, and also in terms of the kind of minds we have due to a common culture and language, means we do indeed perceive the same objects and in some sense the same world, even though 'the act of perception' is something that requires the mind. So this is how idealism defeats the argument of solipsism, that the idealist only knows his ownmind. It is that we don't - we are social and cultural and linguistic beings, who share a set of definitions, associations - indeed this is a large part of what 'culture' means. So consciousness is in that sense a collective. It's obviously differentiated at the individual level, but much of that is due to the consequences of our individualist culture. (
I think this is where Wittgenstein's private language argument applies.)
But note that this also allows for the efficacy of science. As our conscious doings are indeed embedded in cultural and scientific and linguistic conventions, then it's not as if they're private or unique to us; they are 'intersubjectively validated'. But they can be that, without being intrinsically real or possessing inherent reality. Science is still perfectly sound, but it is not underwritten with reference to some purported 'absolute existent'. That's the crucial qualification.