Servetus exhibits in his thesis that "air spirit" (air) is carried by blood from lungs to heart and brain, the same that happened in that the original divine inspiration, the Creation one, the Genesis one, says Servetus. Hence every new inspiration and ulterior illumination stimulates that. In each inspiration “pneuma” or “spirit” enters the lungs. This air, by blood circulation, illuminates the mind when arrives to brain, continuously, blow to blow, breath to breath.
Soul quality depends, at structural explanation, on conformation and disposition of blood vessels in the brain, recognized Servet. At the functional level, however, depends on the quality of “spirit” that enters these vessels. Servetus speaks of bad spirits and good spirits acting in the cavities of the ventricles of the brain. And as pointed out A. Alcalá, with bad spirit Servetus do not mean "evil spirit" or “demon”, but simply notes that the "spirit" (air) can be physiologically and mentally (and morally) either beneficial or harmful. According to this would exist a physical basis, a "good air" and a "bad air", in the root of all mental activity (and also of morally good intentions and bad ones).
What is essential is the air, not blood. The spirits, good or bad, are in the air, or rather, are the air. So much so that Servetus proposes a second route of entry of air-spiritus different of that of the pulmonary circulation of blood. He explains that, in small proportion, the capillary arteries of the choroids, "where the mind is located very safely" when expand absorb "directly" additional air that penetrates through the ethmoid bone.
Needless to say that to current physiology is unacceptable to locate mental functions at the brain ventricles and not at the brain mass. We have seen before that brain mass, to Servet, has a secondary function of pure physical containment and temperature regulation of highly subtilized blood. "The power of the spirit is air," says Servet literally. Moreover, the spirit is air, always defends. Therefore, the air or spirit, when is given in a pure state, necessarily is located in empty spaces, in this case in the hollow cavities of the brain ventricles, where there is no cerebral mass or other form of matter. So there is pure spirit. Says Servetus this air aspired from ethmoid is conducted from the two previous ventricles into the central one. Here, the action of this ventricular pure air is added to the one of the subtle and light air that provides blood of the arteries of the choroids. At the central ventricle, being smaller and also more abundant in vessels of arteries of choroids, is where spiritual light shines more and mind is more lively and understanding more lucid.
The mind, according Servetus, is the ability to think or combine 'something new' (new contents, we would say) that bear some resemblance, mix them, infer one from each other and differentiate each other based on innate ideas or already received images. This ability lies in the functioning of the brain as a “light” or igneous spirit, which feeds on universal spirit (air) and body (blood). The universal spirit, which is God and air at once, literally, lights in us the light of the mind by itself and blood.
It seems an "energetic" theory of the mind. Mental activity is explained as subjective experience of igneous activity (metabolic, we might say) that occurs in brain and nerves, which is fed by the air coming from environment. Mental performance depends primarily of this contribution of air. Possible variations in quality of air regulate quality of metabolic activity and therefore quality of mental activity. The ability to mix, identify and differentiate contents, which is thought, and quality (even moral) of this thinking would depend directly on variations in aspirated air quality, that is the universal spirit of God itself in the christian sense, according Servetus. Thus changes in our mental activity depend on changes that occur in air-spirit-God. This air-spirit-God is dynamic, fluctuates over time and so is also manifested in the actions that result from mental activity. God enlightens us, at times, to find the truth of things, with aeration, through blood, of brain igneous spirit (our own light mind).
Servetus confronts with the difficulty of having to explain how a purely material activity causes mental activity. He escapes, however, of philosophical approaches that attempt to explain what is inexplicable and, far from wanting to explain everything, opens the possibility of a pragmatic and empirical study of mental activity as a manifestation of fluctuations in the air acting on organic systems: respiratory, circulatory and nervous. Servetus basically raised a scientific hypothesis, which still remains open and not studied yet.