We say: among simple actions are those that are performed with the help of one force, for example, digestion, and also those that are performed with the help of two forces, such as the urge to eat: it arises as a result of the natural attractive force and as a result of the sensing force, which acts at the mouth of the stomach. As for the attracting force, it sets the longitudinal fibers in motion, using to the end what they attract, and absorbs the liquid they deliver. And the sensing force manifests itself by conveying the sensation of this influence and irritating the mouth of the stomach with the black bile that stimulates the appetite, which was already discussed earlier.
This action belongs to those that are performed with the help of two forces, only for the reason that if damage occurs to the feeling force, the phenomenon called hunger and appetite disappears, and there is no desire for food, although the body needs it. Swallowing is also accomplished with the help of two forces; one of them is natural attracting, the other is arbitrary attracting. The action of the first is accomplished with the help of longitudinal fibers located at the mouth of the stomach and in the esophagus, and the action of the second is accomplished with the help of fibers of the swallowing muscles. When one of these forces stops working, swallowing becomes difficult. Moreover: if the function of the force does not stop completely, but it is no longer completely devoted to its action, swallowing also becomes difficult. Don't you know that if the appetite changes, it is difficult to swallow food that we do not want. Moreover: if some food is disgusting to us, but we still want to swallow it, and the attractive force of appetite is alienated from this, then it is difficult for the arbitrary attractive force to swallow this food.
The passage of nutrients is also carried out with the help of a force that expels them from the organ from which they are separated, as well as a force that attracts them to the organ to which they are sent. The expulsion of eruptions from both passages occurs in the same way.
Sometimes the source of action is two forces - mental and natural, and sometimes its cause is strength and a certain quality. This is, for example, cooling, which prevents the movement of matter. It helps the expelling force to resist the juices flowing to the organs, retain them and drive them in the right direction.
The quality of cooling is inhibited in two ways essentially, that is, by compacting the outflowing substance and narrowing the pores, as well as in a third way, which is classified as secondary, that is, by extinguishing the attracting heat. And an attractive quality attracts by means opposite to those mentioned. The "compulsion of emptiness" attracts first what is soft and then what is rough; As for the natural attracting force, it attracts what is most suitable, or what it is supposed to attract by its nature. Sometimes the most appropriate thing is the roughest thing.