The parts of a horse, for example, the head, neck, legs, hoofs, tail are not the horse, they are parts of the whole body of the horse. Similarly, the atoms are parts of the horses’ body. They are not in themselves the body of the horse just as my hand is not my body but a part of my body. The substance of a thing is that which ‘stands under’ whatever can be predicated of the thing. The atoms found in the body of a horse are predicated of the whole horse so they can’t be the substance in itself of the horse nor any of its other parts. The form of the whole stands under the parts. This form is called the substantial form of the horse, its horseness, and the principle why the matter of the horse is organized into the body of a horse. The substance of a thing answers the question ‘What is it’. We answer ‘a horse’.