The problem of the unity of consciousness. How could a material object like the brain, extended across space and composed of billions of discrete physical parts, serve as the basis for the unity of our conscious experience?
But now what’s the difference between saying “existence exists” and “the physical world exists”? “Existence exists” does not specify what exists. It is a formula which would cover the first sensation of an infant or the most complex knowledge of a scientist. It applies equally to both. It is only the fact of recognizing: there is something. This comes before you grasp that you are performing an act of consciousness. It’s only the recognition that something exists. By the time you say that it’s a world, and it’s a physical world, you need to know much more. Because you can’t say “physical world” before you have grasped, self-consciously, the process of awareness and have said, “Well, there are such existents as mental events, like thinking or memories or emotions, which are not physical; they are existents, but of a different kind: they are certain states or processes of my consciousness, my faculty of grasping the existence of that outside world.” And the next step is: “What is that outside world made of?”
The concept “matter,” which we all take for granted, is an enormously complex scientific concept. And I think it was probably one of the greatest achievements of thinkers ever to arrive at the concept “matter,” and to recognize that that is what the physical world outside is composed of, and that’s what we mean by the term “physical.” (Ibid., p. 247)