The material brain and the mental brain are two brains that together make one. Thus two, three, four, or innumerable things are yet one thing. The human understanding was endowed by nature with the faculty of embracing the infinite variety of the universe as a unit, as a single conception. The unity of nature is as true and real as its multiplicity. To say that many are one and one many is not nonsense, but simply a truism which becomes clear when understanding the positive outcome of philosophy. A reader unfamiliar with this our product of philosophy still follows the habit of regarding the physical body as something different from the mind. A distinction between these two is quite justified, but this manner of classification must not be overdone. The reader should remember that he also is in the habit of regarding such heterogeneous things as axes, scissors, and knives as children of the same family by referring to them collectively as cutting tools. The outcome of philosophy now demands that we apply the same method to the object of our special study, the human brain. We must henceforth eschew all effervescent flights of imagination and regard the powers of the human mind as children of the same family as all other physical powers, whose immortal mother is the universe. The universe is infinite not alone in the matter of time and space, but also in that of the variety of its products. The human brains which it produces are likewise internally and externally of an infinite differentiation, although this does not prevent them from forming a common group uniform in its way. To group the phenomena of nature, the children of the universe, in such a way by classes, families, and species that they may be easily grasped, that is the task of the science of understanding, the work and constitution of the perceiving human brain. To understand simply means to obtain a general and at the same time a detailed view of the processes and products of the universe by grouping them in a fashion similar to that used for the vegetable kingdom by botany and for the animal kingdom by zoology. It goes without argument that we, the limited children of the unlimited universe, are able to solve this problem only in a limited way. However, this natural physical limitation of the human understanding must not be confounded with the abject misery which slavish and sentimental metaphysics attribute to it. The infinite universe is by no means niggardly in its gifts to the human understanding. It opens its whole depths to our intellectual understanding and perception. Our intellect is a part of the inexhaustible universe and therefore partakes of its inexhaustible nature. That part of nature which is known by the name of intellect is limited only to the extent that the part is smaller than the whole. V THE UNDERSTANDING AS A PART OF THE HUMAN SOUL