Mind aims at the world, which it believes objectively real. Because it sometimes misses, we acknowledge the category of the subjective. But what if there is no reality but that which we impute to experience? What if fundamentally we are left alone with the intention of our aiming, and any apparent target is but some unfolding implication of that intending?
We have already seen that the world-in-itself is unknowable-- by definition untouched by mind-- whereas knowledge, conception, and even perception are the very activities of mind. What can be said about the world-in-itself must be spoken in the language of mathematics. Since there is nothing that can be said about it in the language of experience (or in conceptual domains based upon experience) except that it must exist, the world-in-itself is but a place-holder, to remind us of the fundamental limit and context of mind. Chaos, Void, the Tao are some of its names. Zero is its counterpart in arithmetic, vacuum in physics. Buddhists speak of emptiness, existentialists of nothingness.
Our basic physical experience of space is that it is filled with objects which it separates. Object but no space (the plenum), and space but no object (the vacuum), are logically the same. The essence of physical reality, and therefore the preoccupation of mind, is an inhomogenous world of separation, multiplicity, difference, contrast and change. Where these are not found, or not found sufficiently, our experience is boredom.
Boredom is the mind's rejection of emptiness. But if emptiness is the true nature of reality (even of physical reality), then boredom is the insistence on something which is not real. If what is abidingly real is a stillness without moving parts, then the mind's preoccupation with the mechanics of life is illusory, and its inability to sit still pathological.
What would be a positive experience of emptiness? Though attractive, the idea is subtly paradoxical, for all that mind is conditioned to recognize as experience is a content-- a something-- rather than the nothing of emptiness. Perhaps emptiness cannot be experienced so much as allowed for. We can allow that it is veritable, though transcendent and inaccessible because of the very nature of mind. We can enjoy the blissful experience of turning towards it, but even this is a content, and therefore not "it". Emptiness is another name for the unknowable, the inscrutable world-in-itself, the off-limits for mind.
The picture of reality painted by modern physics is of an insubstantial world. Most of what we consider solid objects consist overwhelmingly of space between elementary particles-- for instance the space between the nucleus of an atom and the vibrating "shell" of electrons. Because even the nucleus itself can be collapsed into a much smaller volume-- perhaps indefinitely-- it consists also of apparently nothing! In outer space, too, the picture is of relatively little content dispersed in vast reaches of empty space. But is space-- the vacuum-- truly empty? To refer again to modern physics, the picture is that space is not empty, but itself seething with potential energy. New particles can appear out of nothing but the energy of space. While matter is essentially vacuum, the vacuum is actually a plenum!
This aspect of physical reality has its parallel in subjective experience. We can know the insubstantiality of perception and thought as mental constructs, as subjective illusion. We can know the truth of the emptiness of our concerns, and of all the movements of mind, of all thoughts including emotion. And far from being distraught because of it, we can also know the seething energy underlying this emptiness and filling the stillness of mind. And finally, then, we can rest in the knowledge that this energy-- even if it be ultimately nothing-- is what we truly are.