As the object of study of the Hippocratic school is body and its diseases, it is not surprising that the Hippocratic physicians defended a physical view of the soul, but interestingly mainly physical rather than bodily. If the pre-Socratic philosophers agreed to say that the air, or anything associated with it, was the universal principle and spiritual element par excellence, or the universal soul or primus motor that said Aristotle, the Hippocratic particularly discussed concretes air conditions such as wind, temperature and humidity, that affect the body and mind and their healthy or pathological functioning.
The fifth section of the Aphorisms of Hippocratic Tractatum alludes to pneuma, that meant "breath" or "air" and spirit or soul too. Hippocratic called pneuma to air inside the body, and just "air" when out of it. Through the mouth, nose, or skin surface, air penetrates the body and becomes pneuma. The functions of pneuma are: feeding, refresh, revive, drive, etc the different tissues and organs. But, pneuma not only acts on bodily functioning. It has a much more decisive importance: its main function in the brain is to produce the understanding and intelligence, higher mental functions or potencies of soul!
The action of pneuma in the brain and mind -or soul- is explained in On Sacred Disease, tractatus dedicated to the study of epilepsy and that is a real theory of the soul:
When man takes air by mouth and nose (breathing), it goes first to the brain, and then mostly towards the belly, and a portion goes to the lung, and another to the veins. From here it disperses to other members through the veins. And whole portion that reaches the belly, that refreshes the belly, it is not good for anything. And so that goes to the lungs. But the air entering the veins is distributed at cavities and the brain, and thus seeks the understanding and movement to members, so that, when the veins are clogged with phlegm and can not receive the air, leave the individual voiceless and reasoning (referring to epileptic seizures). (On Sacred Disease, p 7-10)
The following paragraphs of On Sacred Disease are probably the most beautiful and the most brilliant of the Hippocratic School on the location of mind in the brain. Clearly illustrate the variable nature of mind and the conception according to which mental states fluctuate depending on brain functioning. But even more important, so we are interested, is the explicit and unequivocal statement that is made in the sense that the brain, when healthy, is the interpreter of the stimuli that come from air, and air is so that gives understanding. Thus, intelligence and consciousness become not only from the brain but from the variations of the air, of which the brain is an interpreter:
People should know that our pleasures, joys, laughter and playing do not come from somewhere else but from here (the brain), and so the amount of misery, disappointment and tears. And through it precisely we reason, sense, and see and hear, and distinguish the ugly, the beautiful, the good, the bad, pleasant and unpleasant, and distinguish some things according to the usual standard, and perceive others according to convenience, and thus to distinguish pleasures and dislikes as appropriate times we do not like (always) the same. Because of them we madden and rave, and we face horrors and terrors, at night and other during the day, and insomnia and unwelcome ranting, unmotivated concerns and states of ignorance of the actual circumstances, and oddities. All of that we suffer from the brain, when it is not healthy, it gets hotter or colder, wetter or more dry, or suffers some other condition contrary to its nature. (p 17)
... Accordingly I consider that the brain has the greatest power in man. It is our interpreter, when healthy, of stimuli that come from the air. Air provides understanding. Eyes, ears, tongue, hands and feet execute what the brain perceives. There is understanding throughout the whole body, while there is participation of air, but the brain is the transmitter of consciousness. When man takes inside the air he breathes, it reaches the brain first, and then distributes to the rest of the body, having left in the brain best, and what makes him wise and to have intelligence. If it were first to body and second to brain, after leaving in flesh and veins their power of discernment, it would still warm and already impure, being mixed with the humor of flesh and blood so that would be no longer limpid. Therefore I affirm that the brain is the interpreter of understanding.
... The ability to understand not participate neither, heart nor diaphragm, but responsible for all this is the brain. As is the first organ to perceive intelligence from the air, so, if there is a strong change in the air due to the seasons, and the air itself is altered, the brain is the first organ to perceive. So, I just affirm that the ailments that attack this are the sharpest, the most serious, the most deadly and the most difficult to judge for the inexperienced. This disease, that is called "the sacred disease" (epilepsy), originates from the same causes that others, from cold, the sun and winds changing and never stable. Those are things of God, so that nothing has to distinguish this condition and consider which is more divine than others, but they are all divine and human. (p 9-21)
What is divine in the sacred disease, and also in normal function of brain, is what is natural. The order of nature (cold, sun, wind...), always changing and unstable, governs the functioning of the body and mind of man, and in this sense, this order or logos of nature, as is the cause and origin of physical and mental states, is “divine”.
Hippocrates rejects location of thought and emotion in neither diaphragm nor heart. He reserves to brain this superior paper, which clearly manifests functioning as interpreter of intellection and being the first receiver capable to “understand” data and stimuli that leads the pneuma, or air we breathe, from outside.
The brain is the interpreter, and air flowing through the veins as pneuma, provides information and understanding. Understanding does not "emerge" by magic from the brain, but that understanding lies outside in the changing order of nature, it is in logos (or it is logos) of the natural world. Specifically it is flux and evolution of air. The brain simply is sensitive to air logos, interprets it, physically and biologically, when it reaches from the blood circulation.
The brain, like any other organ in the body, falls ill. And it does that by the same physiological or biophysical mechanism acting when healthy. Brain diseases are neither more nor less divine than the others, but they respond to the same natural causes as other diseases: changes in temperature, humidity... and, above all, in air or wind. In fact, brain diseases, maintain the Hippocratic, are the most serious just because this organ is the first to be affected by "severe change in the air". Atmospheric environmental conditions are responsible for both the general functioning of the organism and the understanding or psyche. Body and mind, in this sense, are the same; they are basically sensitive to logos of nature.
Body responds to atmospheric states and their succession. Brain, in particular, just does it with more finesse, captures more information and more subtle than the other organs, is more sensitive. It is the first organ that reaches the air, in its purest form. It captures small fluctuations of the air that do not capture other organs. And it translates these fluctuations of information or logos in the logos of thought.
Mind is not reduced to structure of brain but is the result of functioning of brain in response to flux of stimuli, especially of pneuma. Thus, physical phenomena and mental phenomena are very similar in nature. There is only a difference of degree in organ sensitivity and purity of pneuma they receive. Thus types of phenomena respond to the same external factors. Bodily manifestations and mental manifestations appear concomitantly. Ones are not the cause of others, don’t determine each other, but are parallel manifestations of the same natural phenomenon. Moreover, to the Hippocratic, mental manifestations are not cause each other, but they are just that, manifestations of a natural phenomenon involving the action of air on the body, primarily on the brain and secondarily on the heart, the diaphragm and other organs. No psychological causes but psychological manifestations. Mental activity is not dependent on the will of the subject but this mental activity, including the own will, is the manifestation of the functioning of a body and a brain sensitive to the action of air logos.
Mental activity is naturally variable, literally as air. We do not like always the same things. We like or not the same as far we perceive and value it differently, we want it or not, we sense and we reason differently about it, just the same ones, without any static control of the process from ourselves, but, on the contrary, we -or our consciousness- are the manifestation of the process.