In other words, the relative and transient forms perceived by our senses serve as raw material for our brain activity, which abstracts the general likeness out of the concrete forms and systematizes or classifies them for our consciousness. The infinite variety of sense perceptions passes in review before our subjective mind, and it constructs out of the multiplicity the unity, out of the parts the whole, out of the phenomena the essential nature, out of the perishable the imperishable, out of the attributes the subject. The essence, the nature of things, the "thing itself" is an ideal, a spiritual conception. Consciousness knows how to make sums out of different units. It can take any number of units for its sums. The entire multiplicity of the universe is theoretically conceived as one unit. On the other hand, every abstract sum consists in reality of an infinite number of sense perceptions. Where do we find any indivisible unit outside of our abstract conceptions? Two halves, four fourths, eight eighths, or an infinite number of separate parts form the raw material out of which the mind fashions the mathematical unit. This book, its leaves, its letters, or their parts, are they units? Where do I begin, where do I stop? In the same way, I may call a library with many volumes, a house, a farm, and finally the whole universe, a unit. Is not everything a part, is not every part a thing? Is the color of a leaf less of a thing than that leaf itself? Perhaps some would call the color simply an attribute and the leaf its substance, because there might be a leaf without color, but no color without a leaf. But as surely as we exhaust a heap of sand by scattering it, just as surely do we remove all the substance of a leaf when we take away its attributes one after the other. Color is only the sum of reactions of leaf, light, and eye, and so is all the rest of the matter of a leaf an aggregate of interactions. In the same way in which our reason deprives a leaf of its color attributes and sets it apart as a "thing itself," may we continue to deprive that leaf of all its other attributes, and in so doing we finally take away everything that makes the leaf. Color is in its nature no less a substance than the leaf itself, and the leaf is no less an attribute than its color. As the color is an attribute of a leaf, so a leaf is an attribute of a tree, a tree an attribute of the earth, the earth an attribute of the universe. The universe is the substance, substance in general, and all other substances are but its attributes. And this world-substance reveals the fact that the nature of things, the "thing itself" as distinguished from its manifestations, is only a concept of the mind.