As for animal force, it means the force that, having arisen in the organs, disposes them to perceive sensations, movements and vital actions. To this are added the movements of fear and anger, for in this case the expansion and contraction that occurs with pneum, associated with this force, arise.
Let us present this general definition in detail and say the following. Just as from coarse juices, due to the influence of a certain nature, a dense substance is born, namely an organ or part of an organ, so from the vaporous and volatile parts of juices, in accordance with a certain nature, a volatile substance, namely pneuma, is born. Just as the liver is, according to doctors, the source of the origin of the first, so the heart is the source of the origin of the second. Pneuma, when it arises from the presence of the nature which it ought to have, is capable of receiving a certain force; This is the force that makes all organs capable of perceiving other forces - mental and other.
Mental forces arise in the pneuma and organs only after the emergence of this force. If an organ has lost its spiritual
strength, but has not yet lost animal strength, then he is alive. Don’t you see that a numb member or a paralyzed member immediately loses the power of sensation and movement, the perception of which is hampered by a painful nature, or a blockage formed between the brain and a given organ in the nerves leading to the organ, but at the same time the member still lives. And an organ that has suffered death loses sensation and movement and undergoes rotting and decomposition. Consequently, in the paralyzed organ there is a force that preserves its life, so that when the obstacle is removed, the power of sensation and movement flows to it and it is able to perceive it, for the animal force in it is healthy; the only obstacle was that which prevented one from actually perceiving this force. But in a dead organ this is not the case.
The giver of this ability is not the feeding force alone, so it cannot be said that while this force remains, the organ is alive, and when it ceases to exist, it is dead. The same reasoning also applies to the feeding force: sometimes its action in some organ ceases, but it remains alive, and sometimes the action of the feeding force remains, but the organ goes to die.
If the nutritive force, since it is a nutritive force, made the organs capable of sensation and movement, then plants would undoubtedly be capable of perceiving sensation and movement. It remains, therefore, to admit that the principle giving this ability is something else; this principle is subject to a special nature and is called animal force. This is the first force that arises in the pneuma, when the pneuma arises from the volatile parts of the juices.
Then, according to the wise Aristotle, the pneuma is directed with this force to the origin and to the first soul, from which other forces spread, but the actions of these forces do not emanate from the pneuma from the very beginning, just as sensations, according to doctors, also do not come from the mental pneuma located in the brain until the pneuma penetrates the skin, tongue and other organs. When a part of the pneuma finds itself in the cavity of the brain, it takes on a nature suitable for the force that resides in it to begin to emanate from the pneuma for the first time through its mediation. The same thing happens in the liver and testicles.
Doctors believe that until the pneuma takes on a different nature in the brain, it is not capable of perceiving the soul, which is the source of sensation and movement. The same thing happens in the liver, although the primary mixing gave the liver the ability to perceive the first animal force. In the same way, for every organ, according to doctors, there is a special soul for each type of action. It is not true that the soul is a single principle from which all actions flow, or that the soul is the aggregate of many souls.
The fact is that if primary nature imparted the ability to perceive the first animal force wherever pneuma and the force that is its perfect manifestation arose, then this force alone, according to doctors, is not enough for pneuma to perceive all other forces through it, until no special nature will arise in her.
Doctors say: this force, along with the fact that it prepares for life, is also the beginning of the movement of the subtle substance of pneuma to the organs and the beginning of its contraction and expansion during inhalation and purification. As they say, this force in relation to life is, as it were, subject to influence, and in relation to the actions of breathing and the beating of the pulse, it itself informs the action. This force is similar to natural forces in that there is no arbitrariness in the actions emanating from it, and it is similar to mental forces in that its actions are diverse, for it simultaneously compresses and expands, that is, it produces two opposite actions. But only the ancient philosophers, calling the earthly soul “soul”, understood the perfection of the natural body, which is an instrument, and meant the beginning of all force, from which, as such, emanate movements and actions that differ from each other. According to the ancients, this power is the power of the soul; the natural strength we mentioned is also called spiritual strength among them.
If we do not give the word “soul” such a meaning, but by it we mean a certain force, which is the beginning of comprehension and movement emanating from it according to some arbitrariness, as a result of some comprehension, and by “nature” we mean any force from which action comes in body in a different way from the above described, then the force that we spoke about will not be mental force, but a natural force, standing at a higher level than the force that doctors call “natural”. If we call “natural force” the force that manages the matter of nutrition and transformation of nutrients - no matter for the sake of preserving the individual or for the sake of preserving the species - then this is not a natural force, but a force of the third kind. Since anger, fear and similar feelings are the result of the action of this force, although their source is sensation, opinion and powers of comprehension, they are attributed to these forces. Checking the presentation of the essence of these forces and establishing whether it is one force or more than one belongs to the science of nature, which is part of philosophy.