We have seen that for Aristotle, the soul is the organizing or unifying principle of a living organism. Now wherever there is unity, there is form. Hence, the soul is the substantial form of the body. This implies, among other things, that it is not a separate substance, as it was for Plato. Rather, body and soul together constitute one living substance. Hence, man is a psychosomatic unity.
Now, we come to understand the nature of a thing through its activities, for a thing acts according to its nature. The acts of a thing reveal the thing's nature. For example, we understand that a plant is a living kind of thing because it grows, among other things. Growth is an activity that belongs to living things. We understand that an entity is an animal because it moves from one place to another and can detect sounds, etc. The kind of activity a thing performs, such as locomotion and hearing, depends upon the kind of thing it is. Plants do not move from one place to another, nor do they detect sounds, for a plant is not an animal, but a plant, and it does not belong to the nature of a plant to be able to run, jump, hear or see, etc.
If a plant is actually growing (activity), then it follows that it has the power (potentiality) to grow. If I am actually typing, it follows that I have the power to type. If you are actually thinking, then it follows that you have the power to think. Activity, in other words, is the realization of a power (potentiality). And so it is the powers of a thing that are the immediate principles of the thing's activity. We call these powers faculties. These faculties or powers are actual potentialities, and so powers do not belong to the material principle of a substance, but to the formal principle of a substance, that is, they inhere in the substantial form of the organism; the substantial form or soul is the principle of actuality of a living thing.
The substantial form of a plant, for example, has three distinct powers that are the proximate principles of the three distinct activities of growth, reproduction, and nutrition.
Now plant life is not the most complex kind of living thing in the physical universe. The next level up on the hierarchy of being is that of animal. This level includes the vegetative, but for Aristotle, this does not necessitate an added soul. An animal is one unified substance, having one substantial form, not two -- otherwise it would be actually two beings. But this one substantial form has many powers. Along with the powers of nutrition, reproduction and growth, the substantial form of a dog, for instance, has the powers of external and internal sensation. But higher than the animal level is the human level, which includes all the levels below it and surpasses them in the two specific powers of intellect and will.