Servetus's work is unique in the attempt to unite two spheres of human reason that are seemingly as far away as beliefs of Christian mysticism on the one hand, and empirical experience and scientific knowledge on the other. In history of philosophy, history of science and history of religions has always been considered that there are two quite distinct and irreconcilable areas of experience. Servetus, exceptionally, unites them. And he did, indeed, from the idea of pneuma, a form of 'spirit' that enters the body through breathing and blood circulation and acts very powerfully on mind. Servetus made an extensive and interesting collection of Christian writings that support this idea. He thinks materialistic as the ancient Greeks, but in his case from the core of Christianity and basing it on the Christian dogmas!
In the first book of De Trinitatis Erroribus Servetus states that Christ is in spirit of God and that it is located in the sky, from the beginning to the end of times, Christian idea of unanimous acceptance. But clarifies on the concept of "spirit of God" or "holy spirit": "...in fact, Scripture on this subject is surprisingly and almost incomprehensible, especially to those unfamiliar with his usual peculiar form of speak, as Holy Spirit they understood God himself, now an angel, now the spirit of man, some instinct or divine breath of the mind, a mental boost or breath away. (...) And some by Holy Spirit do not want to understand anything but the correct intelligence and man's rational capacity. Among Hebrews 'spirit' means nothing but breathing or breath, naming either wind or air, and among the Greeks, by 'pneuma' any means an air and impulse of the mind; and is not an obstacle that the spirit is called "holy", because all the movements of the soul, as concerning the religion of Christ, are rated as holy and consecrated by God."
Servetus matches 'holy spirit' with 'spirit' and points as valid the multiple meanings of it in the different traditions, and collected as the same, or referred to different aspects of the same. The concept of spirit refers to air, wind or divine breath, while movement of the soul, instinct or impulse of the mind, while intelligence and reason, while alludes to the presence of an angel or God himself. All of them are the "spirit". Instead of fragmenting the concept, Servetus maintained, with every intention, a unitary conception of the spirit as main idea of his thesis.
In the second book clarifies that the spirit is in the sky and all over the world, covers everything like an inaccessible blanket of light. Stresses in particular that, in addition to outside, is inside us also. In the same book, reproaches the philosophers who, in their ignorance of the meaning in Scripture, are wrong to underestimate the spiritual nature of the air, that is God's holy spirit and also human spirit which flows through the breath: "... the energy and life-giving spirit of the deity is in the matter we inhale and exhale, as he (God), with his spirit, holds in us the breath of life and gives breath to the people on the earth and spirit to they who run through it, it just moves the skies; bringeth forth the wind out of hiding. (...) To get straight to Holy Spirit we started by the spirit of God. Indeed philosophers, not knowing this energy of the deity, have been unable to understand for what purpose the wind spirit is called spirit of God (Genesis 10). And they do not worry if God sends it to us from the deposits he has or if it pours through him. Therefore they must know that God, himself, is acting within the actual substance of breath. Here it is: God Himself is so present in your mouth, in your spirit, in and out of you as if you could reach out and touch (Acts 17). By agitation of his spirit, are shaked forces of heaven. Orbicularis matter is a dead thing if not agitated by the spirit of God." (References are from Servetus.)
He continues: "All these considerations on the spirit of God are the preliminaries to speak now of the Holy Spirit, as the model of holiness that is applied to the action of the Spirit of God does not imply anything philosophical: indeed, the spirit of God works inside and out, but what sanctifies is what is inside. Therefore keep in mind the difference between breath (or blow) and spirit, it is called a blow when it comes out, but when acts inside and enlightens and sanctifies the spirit of man, is called holy spirit. Indeed, with regard to us do not say we received the blow, but when comes this blow, we receive the spirit, so that, having blown by mouth, Christ says, 'receive holy spirit ' (John 20). Accordingly, conception of the spirit (with respect to the Old Testament) is much more accessible, indeed, the spirit of the power of God can not be known without the instruments which surround their performance."
Servetus says that the action of the spirit of God "implies nothing philosophical". In fact, repeatedly in his work, expressed his desire to quit the habit of always speaking metaphysically, and regrets that the Scripture never considers nature in its materiality. At the end, the action of the Spirit of God, for him, is a simple physical action of air. Later he will discuss in detail the physical (or psychophysical) mechanism of action. Here clarifies yet that blow is the outside air, which in itself, but from God, is just air. But when acting within man "enlightens and sanctifies" his spirit, and then we have to talk of “holy spirit”. Physically we have to talk about a blow or breathe of air, but in terms of the action of this breath on human reason, we must talk of a mental or spiritual phenomenon.
With the air receive the spirit. Thus, in its origin (nature, "universal spirit"…) and at the “final object” (reason, intellect of man…) it can be described as "divine" or "metaphysical", if you will, but the mechanism of action is physical. And we have to know this mechanism if we aspire to know God truly. "The spirit of the power of God can not be known without the instruments by which their performance is surrounded" says Servetus literally.
Also refers to the air, as well as a spirit, as an angel, and said: "... An angel is just a breath of God (Psalm 104, 4) and that is exactly what Hebrew calls breathing and spirit. (P 266)” At the end of Book VI Servetus writes: "... We know it (the spirit), not only because we see the breath, but because we perceive it within us (John 14). And hearing it almost, as Christ says (John 3).”
As is noted in a footnote, Servetus concludes the “resolution on the holy spirit" as follows: "Say, then, that Holy Spirit is a divine agitation in the spirit of man. Thus, what God lights when shake, also sanctifies when illuminate, and needless quiditative definition here, so the word spirit is related to movement, such as impulse, impetus and breathe, and because God, while move them, sanctifies those who believe in Christ: The spirit in man is called holy and that through faith in Christ." According Servetus, we perceive the spirit within us when illuminated by "divine" agitation, which corresponds, as noted before, the genre of material air movement that comes from God. The nature of this phenomenon of illumination of the spirit of man by stirring the air, defends Servetus, is purely empirical, and it is not needed therefore neither essentialist nor metaphysical discussions. What about metaphysical or divine ("holy") comes within the sphere of faith and beliefs (in Christ) of man, and is not part of the actual phenomenon.
In short, the mental and spiritual world of Servetus would consist of moments (or variations) of "enlightenment" of the human mind which obey to fluctuations and air movements. This psychophysical phenomenon, empirical and analyzable, is the central element of scientific Servetus. What surround this phenomenon, indeed, are matters of faith and metaphysics: the first origin of the air, which “comes from God”, and personal faith in Christ, which is “illuminated” by illuminating the mind (in one who has that faith). This is what surrounds the phenomenon, but the phenomenon itself is pure materiality.
Scripture, Servetus notes insistently, does not explicitly mention the nature of this phenomenon, because the plane of Christian discourse is not to "try to natures" but the personal perceptual experience: "...Who feels there is spirit in him speaks of it as if making a distinction point fingers, but this is unknown to philosophers. It is admirable, however, in the highest degree the effectiveness of the provisions of God, as showing the hypostasis of a noticeable being. And the Scripture speaks distinctly on things perceived differently; serving our ability or modes of perception more than our philosophies. But we are crazy to not allow ourselves to be instructed by what fits so familiar to us. Invites us probe and prove ourselves if we perceive the spirit in us more than ask what entity is or what is its nature, as I often have testified that Scripture don’t talk of natures." (Seventh book)