2. As to our knowledge of the material world: whatever exists is itself, an incommunicable, individual substance. To the core of self-sustaining reality, in the oak-tree for instance, other realities (accidents) are added—size, form, roughness, and so on. All oak-trees are alike, indeed are identical in respect of certain constituent elements. Considering this likeness and even identity, our human intelligence groups them into one species and again, in view of their common characteristics, it ranges various species under one genus. Such is the Aristotelean solution of the problem of universals. Each substance is in its nature fixed and determined; and nothing is farther from the spirit of Scholasticism than a theory of evolution which would regard even the essences of things as products of change.