These are the words of Ernst Mach (1838-1916). Amongst physicists, Mach is known as the last anti-atomist, persisting in his view well after the 1905 papers of Einstein. But Mach's views were not philosophically trivial, based on a line of reasoning going back to the Monadology of Leibniz. In his point of view Relationalism was being neglected in favour of the classical reductionism long in vogue.
From The Analysis of Sensations, published in 1897: The popular notion of an antithesis between appearance and reality has exercised a very powerful influence on scientific and philosophical thought. We see this, for example, in Plato's pregnant and poetical fiction of the Cave, in which, with our backs turned towards the fire, we observe merely the shadows of what passes (Republic, vii 1). But this conception was not thought out to its final consequences, with the result that it has had an unfortunate influence on our ideas about the universe. The universe, of which nevertheless we are a part, became completely separated from us, and was removed an infinite distance away.
Mach greatly influenced Einstein's thinking about Relativity. Ironically, the unfinished task of understanding Mach's principle for inertia must bring together both Atomism (in a more monadic guise) and Relativity.