But the physical apparatus by which we sense everyday experience is proprioceptive, which means that what we are actually sensing are the responses of our own cerebro-nervous system.
Let me bring this idea down to earth with an ordinary garden variety illustration.
There is a rose growing in my garden. I say that the rose "is", meaning that it exists and has being; but do I actually know this for a fact? I could be wrong about the species, of course, having only a smattering of botanical knowledge. But a rose by any other name is still a rose to me, and this particular flower happens to exhibit a set of characteristics that matches the description of every other rose in my memory. I also judge the object of my perception to be a "physical reality" because I can support my visual impression of it with related sensory data. But of what are such data composed?
Cupping the delicate blossom in my hand, I study the flower's crimson petals; but the color, shape and texture that I am experiencing are not attributes of the rose itself but of my visual and tactile sensory faculties. The familiar sweet fragrance I sense in its presence is, in actuality, a chemical alteration between my olfactory nerve endings that recalls past encounters with roses from my memory. I stoop to pluck the flower but am stopped by the *****ly thorns of its stem; the pain I feel—a result of the traumatized condition of the nerves in my fingertips when the skin is pierced—is a further reminder that, except for the presumed beingness of this living plant before me, all of its identifiable attributes are actually properties of my organic sensibility. Thus, the flower whose existence I so confidently and without hesitation reported a moment ago on analysis turns out to be the mere spectre of a rose—a concoction of my own proprietary awareness. I do not even know for a certainty that what I've called a rose has a being of its own that is distinct from my cognizance of it!
Now I may try to corroborate the existence of the rose by inviting my wife to the garden and asking her to confirm it. But what she perceives will be her own set of sensory data relative to what is, in effect, another experience. Since I have no direct access to her sense impressions or values, we can only compare our observations verbally, in a very general way. Likewise, the form of the rose and its physical position among the other plants in the garden may be described, sketched or photographed to provide additional evidence of its existence. But these abstract "proofs" do not validate its "being" any more than self-awareness validates my own. Physical things like houses and stones—even living trees and flowers—are dimensional phenomena that relate to space and time in an objective world, not to being as such. Their supposed beingness is a consequence of their being experienced. And the tools we employ to confirm their existence will always produce data consistent with our experience because that is what they were designed to do.