
Descartes, in his The Passions of the Soul (1649), famously conjectured that the pineal gland, shown above, is the "principle seat of the soul", in the sense of being the thing which allows a person, in theory, to move themselves, in a manner different than as compared to an automaton.

Descartes, in his The Passions of the Soul (1649), famously conjectured that the pineal gland, shown above, is the "principle seat of the soul", in the sense of being the thing which allows a person, in theory, to move themselves, in a manner different than as compared to an automaton.

In geniuses on, Descartes on the soul refers to the ideas, comments, theories, and opinions of Rene Descartes on the topic of the soul, generally discussed in respect to its purported existence in vegetables, animals, and humans, in respect to principles of motion.

Overview
In 1598, when Descartes was age two, Tommaso Francini and Alessandro Francini, Florentine hydraulics engineering brothers, built, in garden terraces at Saint-Germain-en-Laye, Paris, a number of water-driven automata, that turned around, played music, and even one where a hero slayed a rising dragon. (Ѻ) Descartes, as he grew up, was greatly intrigued by these automaton, in respect to their philosophical implications

In 1633, Descartes, aged 37, in his Treatise on Man, 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. [2] The following is the main statement:

“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 [soul] 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 [3]

In 1649, Descartes, in his The Passions of the Soul, his last published book, gives his most extensive account (Ѻ), of his theory that the soul resides in the pineal gland; the following is the main quote: [1]

“My view is that this [pineal] 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.”

(add)

Quotes
The following are related quotes:

“Descartes saw that souls cannot at all impart force to bodies, because there is always the same quantity of force in matter. Yet, he thought that the soul could change the direction of bodies. This was, however, because at that time the law of nature, which affirms also the conservation of the same total direction in the motion of matter, was not known. If he had known that law, he would have fallen upon my system of pre-established harmony. According to this system bodies act as if (to suppose the impossible) there were no souls at all.”
— Gottfried Leibniz (1714), Monadology (pg. 265-66)

“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)

References
1. (a) Descartes Rene. (1649). The Passions of the Soul (Les Passions de l’ame). Art. VI, Oeuvres, Cousin, IV, 41.
(b) Chalmers, David. (2002). Philosophy of the Mind (pg. #). Oxford University Press.
(c) Pineal gland – Wikipedia.
2. (a) Automaton – Wikipedia.
(b) Johnstone, James. (1921). The Mechanism of Life in Relation to Modern Physical Theory (pgs. 153-54). Longmans, Green & Co.
3. (a) Tyndall, John. (1874). “Address” (pg. 20), Delivered before the British Association assembled at Belfast. Longmans, Green, and Co.
(b) 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.