In existographies, Rene Descartes (1596-1650) (IQ:195|#13) (Cattell 1000:23) [RGM:33|1,500+] (Murray 4000:6|CS / 7|M / 4|WP) (Gottlieb 1000:25) (GPE:13) (GME:6) (Library:2,000) (GPhE:3) [CR:441] was a French physicist, philosopher, and mathematician, aka the “restorer of modern philosophy” (Holbach, 1770), noted for his 1637 Discourse on Method, which introduced a number of topics, such as the Cartesian coordinate system, automaton theory, vis viva discussions, and the proof that one exists, embodied in the famous saying “I think, therefore I am”.

Conservation of quantity of motion | Vis viva
In circa 1640 pre vis viva ideas in which he ruminated on the issue of what happens when a body, such as a hard spherical ball, of mass m and speed v collides with another such body of different mass and speed.

“It is obvious that when god first created the world, he not only moved its parts in various ways, but also simultaneously caused some of the parts to push others and to transfer their motion to these others. So, in now maintaining the world by the same action and with the same laws with which he created it, he conserves motion; not always contained in the same parts of matter, but transferred from some parts to others depending on the ways in which they come in contact.”
— Rene Descartes (1644), Principles of Philosophy (pg. 62) [22]

“Motion, indeed, is only a state of the moving body; but it has a certain definite ‘quantity’, and it is readily conceived that this quantity may be constant in the universe as a whole, while varying in any given part. We must reckon the ‘quantity of motion’ in two pieces of matter as equal if one moves twice as fast as the other, and this in turn is twice as big as the first; again, if the motion of one piece of matter is retarded, we must assume an equal acceleration of some other body of the same size.”
— Rene Descartes (1644), Principles of Philosophy (§:36) [23]

He concluded that the quantity mv (now called momentum) would be conserved. In other words, according to Descartes, for two colliding spheres the sum:

$m_1 v_1 + m_2 v_2 \,$

would be the same before and after the collision. In short, Descartes argued for a principle of conservation of motion, where quantity of the product of the mass of a moving object by its velocity mv was thought to be conserved in mechanical interactions. Descartes’ reasoning, however, was partly metaphysical and therefore not convincing. [2]

Education
In 1618, Descartes, aged 22, met and was mentored in mechanical philosophy by Isaac Beeckman.

Vacuum
In 1644, Descartes, in his Principles of Philosophy (part 2, number 16) (Ѻ), declared, like Aristotle, that a vacuum does not exist, via some type of extension of bodies argument. [21] The following is one synopsis take on this:

Descartes tried to explain all physical phenomena in terms of ‘extension’ and ‘motion’ only. Like a Cheshire Cat vanishes leaving only the abstract grin, so in Descartes’ system specific substances and qualities vanish, leaving only a real world of extension and motion. A void or vacuum is impossible for Descartes as it would imply extension without body and for him extension is body. To account for the world as we know it Descartes requires three types of particles, which are distinguished from one another by their geometrical properties only—that is by ‘extension’. These are the small ‘fire particles’, some of them infinitely small, the intermediate ‘boules’ and the ‘gross material particles’, which in aggregate constitute all material bodies. The first two sorts of particles form the all-pervading aether which circulates in a gigantic but invisible whirlpool or vortex about that large hot body, the sun. It is the motion of this vortex that carries the planets round in their circular orbits, which has its own relatively small vortex, that accounts for the phenomenon we call ‘weight’.”
Donald Cardwell (1971), From Watt to Clausius (pgs. 2-3)

Descartes, said another way, held the view that there could be no void, no vacuum, because empty space could only be conceived in terms of matter, which is an extension. [20]

The World
In 1634, Descartes was ready to publish Treatise on the World and on Light (Traité du monde et de la lumière), a book he had written between 1629 and 1633, which was controversially Copernican based, but upon hearing that Galileo was condemned by the Church, decided to suspend publication, so to keep his liberty. [16]

Automatons | Humans
In 1598, Tommaso Francini and Alessandro Francini, Florentine hydraulics engineering brothers, built a number of water-driven automata, in garden terraces at Saint-Germain-en-Laye, Paris, that turned around, played music, and even one where a hero slayed a rising dragon. (Ѻ) Descartes was greatly intrigued by these automaton, in respect to their philosophical implications; Julien Musolino (2015) summarizes this as follows: [19]

“Descartes’ mechanical thinking was heavily influenced by the automata at the Royal Gardens in Saint-Germain-en-Laye, a suburb of Paris. These hydraulically powered statues could perform all kinds of impressive tricks, and they served as the model that Descartes used to describe the functioning of the human body.”

Descartes famously posited that a human was an automaton, a type of spring-loaded machine, similar to animals, except in one respect, that it contained a "rational soul" that could initiate volition of the body by its own accord. [4]

“These functions [digestion, beating of heart, growth, respiration, waking, sleeping, etc.] follow in this [human] machine simply from the disposition of the organs as wholly naturally as the movements of a clock or other automaton follow from the disposition of its counterweights and wheels. As far as these functions are concerned, it is not necessary to conceive any other vegetative or sensitive soul, nor any other principle of motion or of life, than the blood and the spirits agitated by the fire which burns continually in the heart, and which is in no wise different from the fires which exist in inanimate bodies.”
— Rene Descartes (1633), Treatise on Man [13][19]

In c.1949, John Neumann picked up on Descartes's human automaton theories (see: Neumann automaton), in some sense, in terms of free energy. [1]

Death
Descartes, similar to Thomas Hobbes, compared death to the stopping of a watch mechanism. [8] The following is Descartes’ view on the matter: [9]

“The body of a living man (see: alive; living matter) differs from that of a dead man (see: dead; dead matter) only as much as a watch or any other automaton when they are wound up differ from the same watch or automaton when they are broken.”

Treatise on Man
Descartes’ circa 1633 Treatise on Man describes the human body as a machine, and investigates the movement of the blood, animal spirits, the senses, hunger and thirst, digestion, and the brain and its functions. In this work, it is said that he championed the full reduction of the human body to the phenomenon of matter in motion. [5]

Of note, originally Descartes had spent four years, in the 1630s, preparing a double treatise on “man” and “light”, but withheld it from publication on learning how Italian physicist Galileo Galilei, in 1633, had been punished by the church for his advocacy of the Copernican doctrine of the motion of the earth. Consequently, Descartes’ treatise on humans was published post humorously in 1662. [6]

The idea of the body as a kind of animal machine that functions according to physical laws was an immense advance over the previous scholastic notions based on Aristotle, which merely begged the question of how the various organs of the body work by stating that it is in their nature to perform their specific functions.

Cartesian system
Descartes is also the eponym of the Cartesian coordinate system, a very prominent mathematical tool used in thermodynamics, which was first introduced in one the appendices, entitled “On Geometry”, to his famous 1637 Discourse on Method.

Heat
Descartes' 1637 Discourse on Method contains a short chapter on “What do heat and light of fire consist in?”. Descartes believed that heat was a free-floating "ethereal" substance that had neither weight nor mass and that cold was caused by the recess receding that of ethereal substance, which agitated the little eel-like particles of bodies. [3]

 Descartes' 1644 effluvia models of the magnetism the earth (top) and a permanent magnet (bottom). [12]
Chemical bonding
In circa 1625, Descartes was said to have used the hooked atom logic of atomic theory in which "hood-and-eye model of atomic bonding", whereby a bond was said to form when the hook of one atom got caught in the eye of another atom.

Magnetism
In 1644, Descartes published his effluvia theory of magnetism—supposedly based on Empedocles' theory of effluvia—which seems, in some way, to have been a possible precursor to James Maxwell’s 1873 electromagnetic theory. [12]

Atomic theory
In 1637, Descartes became one of the few scientists to revive Greek philosopher Leucippusatomic theory, along with French mathematician Pierre Gassendi (1649), English physicist Robert Boyle (1661), and English physicist Isaac Newton (1686).

Irish physicist John Tyndall (1874), to note, states that Descartes rejected the Greek notion of the atom, because, supposedly, he argued, it was absurd to suppose that God, if he so pleased could not divide an atom; Descartes, says Tyndall, puts in place of the atoms small round particles and "light splinters", out of which he builds the organism. [13]

Free will
From various unpublished manuscripts, it has been surmised that Descartes worried the eventuality of mind initiating motion would jeopardize his conservation principle and his entire mathematical account of motion. [7] Descartes, in short, realized that his conservation of momentum principle conflicted with the doctrine of free will and internal choice.

Soul
See main: Descartes on the soul
Descartes was an early soul theorist, noted for his opinion that the soul is located in the pineal gland: [10]

“My view is that this gland is the principal seat of the soul, and the place in which all our thoughts are formed. The reason I believe this is that I cannot find any part of the brain, except this, which is not double. Since we see only one thing with two eyes, and hear only one voice with two ears, and in short have never more than one thought at a time, it must necessarily be the case that the impressions which enter by the two eyes or by the two ears, and so on, unite with each other in some part of the body before being considered by the soul. Now it is impossible to find any such place in the whole head except this gland; moreover it is situated in the most suitable possible place for this purpose, in the middle of all the concavities; and it is supported and surrounded by the little branches of the carotid arteries which bring the spirits into the brain.”

American physical organic chemist George Scott seems to think that this was Descartes' location for free will. [11]

 A depiction of Christina Alexandra, Queen of Sweden, left, being tutored by Rene Descartes, right, on his "mechanical philosophy"; views for which he was "put to death" (Holbach, 1770) for, i.e. poisoned as it has been surmised.
Reaction end | Poisoned?
There seem to be some peculiar circumstances behind Descartes reaction end; the “official” version being that, at age 54, he dies from pneumonia in Sweden; the unofficial version being that he was poisoned by an arsenic-laced communion wafer by priests fearing of the effect of his materialistic philosophy on the queen of Sweden:

“Descartes symptoms include: weakening, vomits, diarrhea, dizziness, skin's pigmentation, cutaneous damages, enteritis ... symptoms commonly found in an arsenic intoxication.”
— Johann van Wullen (c.1650), “Letter to Willem Piso” [17]

“He expiated his rivals attacks with the innocency of his life.”
— Hector Chanut (1650), words engraved on Descartes tombstone [17]

Virgil, the bishop of Saltzburg, was condemned by the church, for having dared to maintain the existence of the antipodes. All the world is acquainted with the persecutions which Galileo suffered for pretending that the sun did not make its revolution round the earth. Descartes was put to death in a foreign land.”
Baron d’Holbach (1770), The System of Nature (pg. 284)

In 2009, Theodore Ebert, in his The Enigmatic Death of Rene Descartes, conjectured that Descartes was poisoned by an arsenic-laced communion wafer; the abstract of which is as follows: [18]

“This monograph discusses the illness and death of René Descartes. All the hitherto available documents on his illness and death are collected in the appendix, partly also in the original French or Latin. These documents make it rather unlikely that Descartes died of pneumonia, the circumstances of his death suggest a poisoning by arsenic. The possible murderer and his motives are also discussed.”

Quotes | On
The following are quotes on Descartes:

“The embarrassments which animals have thrown in the way of the partisans of the doctrine of spirituality is well known: they have been fearful, if they allowed them to have a spiritual soul of elevating them to the condition of human creatures; on the other hand, in not allowing them to have a soul, they have furnished their adversaries with authority to deny it in like manner to man, who thus finds himself debased to the condition of the animal. Theologians have never known how to extricate themselves from this difficulty. Descartes fancied he solved it by saying that beasts have no souls, are mere machines. Nothing can be nearer the surface than the absurdity of this principle. Whoever contemplates nature without prejudice, will readily acknowledge, that there is no other difference between the man and the beast than that which is to be attributed to the diversity of his organization.”
Baron d’Holbach (1770), The System of Nature (pgs. 81-82)

“It is, indeed, a very observable fact, that while Richelieu, with such extraordinary boldness, was secularizing the whole system of French politics, and by his disregard of ancient interests, was setting at naught the most ancient traditions, a course precisely similar was being pursued, in a still higher department, by a man greater than he; by one, who, if I may express my own opinion, is the most profound among the many eminent thinkers France has produced. I speak of Rene Descartes, of whom the least that can be said is, that he effected a revolution more decisive than has ever been brought about by any other single mind. He was the first who successfully applied algebra to geometry; that he pointed out the important law of the sines that in an age in which optical instruments were extremely imperfect, he discovered the changes to which light is subjected in the eye by the crystalline lens ; that he directed attention to the consequences resulting from the weight of the atmosphere, and that he, moreover, detected the causes of the rainbow, that singular phenomenon, with which, in the eyes of the vulgar, some theological superstitions are still connected. At the same time, and as if to combine the most varied forms of excellence, he is not only allowed to be the first geometrician of the age, but, by the clearness and admirable precision of his style, he became one of the founders of French prose. ”
Henry Buckle (1856), History of Civilization, Volume One (pg. 417)

“Mindful of Galileo's condemnation, Descartes, who had no taste for martyrdom, was always careful, in defending the Copernican system, to refer to it as a ‘fable’ or mere ‘hypothesis’, which, indeed, agreed with the known facts better than any other, but need not on that account be regarded as true; but no reader can have been in doubt either as to the logical outcome of the philosopher's arguments or as to his actual opinion.”
Arthur Lovejoy (1933), The Great Chain of Being (pg. 123)

“Most seventeenth-century scientists accepted Descartesmechanical philosophy’, in which the universe was composed of matter and motion, and all natural philosophy could be explained by the collisions between particles.”
— Stephen Inwood (2002), The Man Who Knew Too Much (pg. 15)

Quotes | By
The following are noted quotes:

“Give me matter and motion, and out of them I will build the universe.”
— Rene Descartes (c.1630), Publication (Ѻ); cited by Ludwig Buchner (1855) in Force and Matter (pg. 64)

“It is now some years since I detected how many were the false beliefs (see: unlearn) that I had from my earliest youth admitted as true and how doubtful was everything I had since constructed on this basis.”
— Rene Descartes (1641), Mediations on the First Philosophy in Which the Existence of God and the Distinction Between Mind and Body are Demonstrated [14]

I am a thing that thinks: that is to say, that doubts, affirms, denies, that wills, that desires, that also imagines and perceives.”
— Rene Descartes (1641), Mediations on First Philosophy (Mediation Three) (Ѻ); note: Hobbes objected (Ѻ) to this [15]; cited by Henry Buckle (1856) in History of Civilization, Volume One (pg. 425)

“If you would be a real seeker of truth, you must at least once in your life doubt, as far as possible, all things.”
— Rene Descartes (1644), Principles of Philosophy (Ѻ)(Ѻ)

References
1. (a) Descartes, Rene. (1637). Discourse on Method. Penguin.
(b) Discourse on Method – Wikipedia.
2. Laidler, Keith J. (2002). Energy and the Unexpected (pg. 22). Oxford University Press.
3. Shachtman, Tom. (1999). Absolute Zero and the Conquest of Cold (pg. 31). Mariner Books.
4. (a) Automaton – Wikipedia.
(b) Johnstone, James. (1921). The Mechanism of Life in Relation to Modern Physical Theory (pgs. 153-54). Longmans, Green & Co.
5. Mirowski, Philip. (1989). More Heat than Light: Economics as Social Physics, Physics as Nature’s Economics (pg. 121). Cambridge University Press.
6. (a) Descartes, Rene and Hall, Thomas S. (2003). Treatise on Man (xii). Prometheus Books.
(b) Descartes, Rene. (c.1633). Traite de l’Homme. Unpublished Manuscript.
7. (a) Gabbey, Alan. (1985). “The Mechanical Philosophy and Its Problems”, in: Change and Progress in Modern Science, J. Pitt, ed., Boston: Reidel.
(b) Mirowski, Philip. (1989). More Heat than Light: Economics as Social Physics, Physics as Nature’s Economics (pg. 122). Cambridge University Press.
8. Sorokin, Pitirim. (1928). Contemporary Sociological Theories (thermodynamics, pgs. 25-27; human molecules, pg. 46-47). Harper & Brothers.
9. Descartes, Rene. (1649). “Les Passions de l’ame” ("Passions of the Soul"), Art. VI, Oeuvres, Cousin, IV, 41.
10. (a) Descartes Rene. (1640). "The Passions of the Soul"; excerpted from "Philosophy of the Mind", Chalmers, D. New York: Oxford University Press, Inc.; 2002.
(b) Pineal gland – Wikipedia.
11. Scott, George P. (1985). Atoms of the Living Flame: an Odyssey into Ethics and the Physical Chemistry of Free Will (pg. 30). University Press of America.
12. (a) Descartes, Rene. (1644). Principia Philosohiae (diagram, pg. 274) (part IV, articles 133-188). Publisher.
(b) Fara, Patricia. (2005). Fatal Attraction: Magnetic Mysteries of the Enlightenment (pg. 130-31). MJF Books.
(c) Baigrie, Brian S. (2007). Electricity and Magnetism: A Historical Perspective (pg. 16). Greenwood Publishing Group.
13. Tyndall, John. (1874). “Address” (pg. 20), Delivered before the British Association assembled at Belfast. Longmans, Green, and Co.
14. (a) Descartes, Rene. (1641). Mediations on the First Philosophy in Which the Existence of God and the Distinction Between Mind and Body are Demonstrated. Publisher.
(b) Descartes, Rene. (1974). Philosophical Works, Volume One (pg. 144). Cambridge University Press.
(b) Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (pg. 316). HarperOne.
15. (a) Descartes, Rene. (1641). Mediations on First Philosophy: with Selections from the Objections and Replies (translator and editor: John Cottingham; introduction: Bernard Williams) (pg. 24). Cambridge University Press.
(b) Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (pg. 317). HarperOne.
16. (a) The World (Descartes) – Wikipedia.
(b) Hecht, Jennifer M. (2003). Doubt: A History: The Great Doubters and Their Legacy of Innovation from Socrates and Jesus to Thomas (pg. 318). HarperOne.
17. Correas, Antonio. (2007). “Why Did Descartes Die?” (Ѻ), Apr 22.
18. Ebert, Theodore. (2009). The Enigmatic Death of Rene Descartes (Der rätselhafte Tod des René Descartes) (Ѻ), Publisher.
19. Musolino, Julien. (2015). The Soul Fallacy: What Science Shows We Gain from Letting Go of Our Soul Beliefs (foreword: Victor Stenger) (Gardens, pg. 46, wheels, pg. 47). Prometheus.
20. Johnstone, James. (1921). The Mechanism of Life in Relation to Modern Physical Theory (pg. 161). Longmans, Green & Co.
21.
Guericke, Otto and Schott, Kaspar. (1672). Otto Guericke’s New Experiments: on (as they are called) on the Magdeburg vacuum space (Ottonis De Guericke Experimenta Nova (ut vocantur) Magdeburgica de Vacuo Spatio) (preface, pdf) (pg. 87). Janssonius a Waesberge.
22. (a) Descartes, Rene. (1644). Principles of Philosophy: Translated with Explanatory Notes (translators: Valentine Miller and Reese Miller) (pg. 62). Springer, 2012.
(b) Rogers, Ben. (2018). The Big Ideas in Physics and How to Teach Them (§: Kinetic Energy and Potential Energy: Descartes and Leibniz: 1644-1676, pgs. 70-). Routledge.
23. (a) Descartes, Rene. (1644). Principles of Philosophy (Principia Philosophiae); in: Oeuvres de Descartes, Volumes Eight (of 13) (editors: Charles Adam and Paul Tannery) (pg. 61). Cerf, 1913.
(b) Litis, Carolyn. (1971). “Leibniz and the Vis Viva Controversy” (abs), Isis, 62(1): 21-35.
(c) Achinstein, Peter. (2004). Science Rules: a Historical Introduction to Scientific Methods (§3: Descartes’ Laws of Motions, pgs. 40-). Publisher.