Harvey then discusses at some length the Aristotelian and scholastic views of the word "spirit" and "vital principle," and in the end arrives at the conclusion that "the blood, by reason of its admirable properties and powers, is 'spirit.' It is also celestial; for nature, the soul, that which answers to the essence of the stars is the inmate of the spirit, in other words, it is something analogous to heaven, the instrument of heaven, vicarious of heaven.... The blood, too, is the animal heat in so far namely as it is governed in its actions by the soul; for it is celestial as subservient to heaven, and divine because it is the instrument of God the great and good."
Harvey next attacks the doctrine of those who maintained that nothing composed of the elements can show powers superior to the forces exercised by these unless they at the same time partake of some other and more divine body, and on this ground conceive the spirits they evoke as constituted partly of the elements, partly of a certain ethereal and celestial substance. He observes very pertinently in opposition to such a train of reasoning: "In the first place you will scarcely find any elementary body which in acting does not exceed its proper powers; air and water, the winds and the ocean, when they waft navies to either India and round this globe, and often by opposite courses, when they grind, bake, dig, pump, saw timber, sustain fire, support some things, overwhelm others, and suffice for an infinite variety of other and most admirable offices--who shall say that they do not surpass the power of the elements? In like manner what does not fire accomplish? In the kitchen, in the furnace, in the laboratory, softening, hardening, melting, subliming, changing, in an infinite variety of ways! What shall we say of it when we see iron itself produced by its agency?--iron 'that breaks the stubborn soil and shakes the earth with war'! Iron that in the magnet (to which Thales therefore ascribed a soul) attracts other iron, 'subdues all other things and seeks besides I know not what inane,' as Pliny says; for the steel needle only rubbed with the lodestone still steadily points to the great cardinal points; and when our clocks constantly indicate the hours of the day and night, shall we not admit that all of these partake of something else, and that of a more divine nature than the elements? And if in the domain and rule of nature so many excellent operations are daily effected, surpassing the powers of the things themselves, what shall we not think possible within the pale and regimen of nature, of which all art is but imitation? And if, as ministers of man, they effect such admirable ends, what I ask may we not expect of them, when they are instruments in the hand of God?