Thinking is a physical process and it cannot exist or produce anything without materials any more than any other process of labor. My thought requires some material which can be thought of. This material is furnished by the phenomena of nature and life. These are the concrete things. In claiming that the universe, or all things, may be the object of thought, we simply mean that the materials of the thought process, the objects of the mind, are infinite in quantity and quality. The materials which the universe furnishes for our thought are as infinite as space, as eternal as time, and as absolutely manifold as the nature of these two forms of being. The faculty of thought is a universal faculty in so far as it enters into relations with all things, all substances, all phenomena, and thus generates thought. But it is not absolute, since it requires for its existence and action the previous presence of matter. Matter is the boundary, beyond which the mind cannot pass. Matter furnishes the background for the illumination of the mind, but is not consumed in this illumination. Mind is a product of matter, but matter is more than a product of mind, being perceived also through the five senses and thus brought to our notice. We call real, objective products, or "things themselves" only such products as are revealed to us simultaneously by the senses and the mind. Reason is a real thing only in so far as it is perceived by the senses. The perceptible actions of reason are revealed in the brain of man as well as in the world outside of it. For are not the effects tangible by which reason transforms nature and life? We see the successes of science with our eyes and grasp them with our hands. It is true that science or reason cannot produce such material effects out of themselves. The world of sense perceptions, the objects outside of the human brain, must be given. But what thing is there that has any effects "in itself?" In order that light may shine, that the sun may warm, and revolve in its course, there must be space and other things which may be lighted and warmed and passed. In order that my table may have color, there must be light and eyes. And everything else which my table is besides, it can be only in contact with other things. Its being is just as manifold as those various contacts or relations. In short, the world consists only in its interrelations. Any thing that is torn out of its relations with the world ceases to exist. A thing is anything "in itself" only because it is something for other things, by acting or appearing in connection with something else.