When it was necessary he did not hesitate to experiment upon himself, for he says: "I have myself, for experiment's sake, occasionally pricked my hand with a clean needle, and then having rubbed the same needle on the teeth of a spider, I have pricked my hand in another place. I could not by my simple sensation perceive any difference between the two punctures: nevertheless there was a capacity in the skin to distinguish the one from the other; for the part pricked by the envenomed needle immediately contracted into a tubercle, and by and by became red, hot, and inflamed, as if it collected and girded itself up for a contest with the poison for its overthrow." The seventy-first essay of the treatise of Development is a good example of the mystic or philosophical side of Harvey's character. The essay is entitled "Of the innate Heat." It begins, "As frequent mention is made in the preceding pages of the calidum innatum or innate heat, I have determined to say a few words here, by way of dessert, both on that subject and on the humidum primigenium or radical moisture, to which I am all the more inclined because I observe that many pride themselves upon the use of these terms without, as I apprehend, rightly understanding their meaning. There is, in fact, no occasion for searching after spirits foreign to or distinct from the blood; to evoke heat from another source; to bring gods upon the scene, and to encumber philosophy with any fanciful conceits. What we are wont to derive from the stars is in truth produced at home. The blood is the only calidum innatum or first engendered animal heat." Harvey then proceeds to examine the evidence for a spirit different from the innate heat, of celestial origin and nature, a body of perfect simplicity, most subtle, attenuated, mobile, rapid, lucid, ethereal, participant in the qualities of the quintessence. Of this spirit Harvey confesses that "we, for our own parts, who use our simple senses in studying natural things, have been unable anywhere to find anything of the sort. Neither are there any cavities for the production and preservation of such spirits, either in fact or presumed by their authors." 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."