IN Order to put an End to the Controversies about the Seat of a Pestilence, which have from Antiquity even to this Day been warmly maintained, many Authors putting the Heart for the Principle of Life and Death, some the Brain, and others the Stomach, Lungs, or Liver: It will be necessary here to discover the immediate Residence of the pestilential _Seminium_. Since therefore the above-mentioned _Aura_, according to Hypothesis, is very subtile and spirituous, for that Reason there must necessarily be some conformable Property in the Matter which is fit to receive it; as therefore there is not in the whole humane Machine any Subject more apposite, and capable of its Union, than the animal Spirits, we must fix its Residence there. But because I am sensible what Objections this Opinion lies open to, with some Persons, who may not conceive how an immediate Infection of the Spirits is communicable to the Viscera, and all Parts of the Body, it will be necessary to go thro’ this Matter in a very particular Manner, by enquiring; _FIRST_, What are the Spirits concerning which we are here speaking? _SECONDLY_, What is that Disposition of Spirits which makes them fit to receive the pestilential Impression? And, _THIRDLY_, After what Manner the vitiated Spirits can affect the whole Body with Disorders? TO this Purpose we must know, that the Spirits are the most thin and subtile Particles of the Aliment and other Juices, raised to the utmost Perfection and Volatility by the innate Heat, and the nitro-aerial Spirit, to serve in the Operations of the Mind, and all the Purposes of the animal OEconomy. THE Matter whence the Spirits are generated is the Chyle, and their Restauration, Confirmation, and Vigour, from the Recruits of Food, as is their Languor, Prostration, and utter Extinction from the Want of it; so that howsoever they were first generated in the original, they owe their Conservation and Vitality to the Nourishment continually brought in; and although in a State of perfect Health _they_ are never changed by _that_, yet they continually act upon that after various Ways, bringing it from a crude, recrementicious State, into a noble Juice, or rich spiritual Balsam, retaining its ideal Character: And hence it comes about, that although there is a daily Waste of Spirits, there is no Want, because Nature is continually, while Things are in Health, making more; insomuch that after a due Constitution of Spirits is obtained, they of themselves are the main Efficients in making more, as one Light is kindled by another, and as the Blood it self is the chief Instrument in Sanguification, or making more Blood.