"Whatever our philosophical standpoint may be, for all purposes of scientific observation an object exhausts itself in the totality of possible relations to the perceiving subject or instrument. Of course, mere perception does not constitute knowledge and insight; it must be coordinated and interpreted in reference to some underlying entity, a "thing in itself," which is not an object of direct physical observations, but belongs to metaphysics."
What does the author mean by "underlying entity"? - Is it what the object is made off? What is inside the object?