Theories about the anatomical location of the spiritual component of the human being have their roots in the earliest manifestations of philosophical thought and the emergence of religions, although the scientific community has not been absent from the development of this debate (López, Rubio, Molina & Alamo, 2012). Indeed, one of the most interesting hypotheses regarding the corporeal seat of the human soul was proposed by René Descartes. He argued that the compact structure of the pineal gland, like the structure of other parts of the brain, is composed of fibers separated by pores through which the blood, coming from the choroid plexuses and the epiphyseal arterioles, penetrates the brain. According to López, Alamo & García (2010), for its function, the pineal gland extracts fine particles suspended in the blood and produced in the left ventricle of the heart, under the heat of the myocardium, and transforms them into animal spirits.
On the other hand, those coarser particles, not filtered by the pineal gland, enter the cortical surface of the brain and serve as nutrients (López, Alamo & García, 2010). These spirits have two fundamental properties: they are extremely small particles and move with great speed. Thus, according to López, Alamo & García (2010), Descartes gives the pineal gland a primary and basic role in the intimate mechanism of the relationship between man, machine, and his environment.
The Importance of the Pineal Gland Throughout his Works
Descartes' assertion that the pineal gland is the main seat of the soul has stood out as one of the most enduring and significant statements of his thought (Shapiro, 2011). This assertion remains present in his biological writings and extends to his work "The Passions of the Soul." In "Treatise of Man," he first associates the soul with the pineal gland specifically and directly (Shapiro, 2011). For Descartes, the pineal gland played a fundamental role, as it was involved in processes as diverse as sensation, imagination, memory, and the causation of bodily movements (Lokhorst, 2013). However, according to Lokhorst (2013), it should be noted that some of Descartes' basic anatomical and physiological assumptions proved to be completely wrong, not only by current scientific standards but also in light of the knowledge available in his own time.
In the "Fifth Discourse on Optics," he reiterates the soul's connection with this specific part of the brain (Shapiro, 2011). This same idea is revisited in the "Meditations," where he insists that the soul is not immediately affected by all parts of the body, but only by the brain, or perhaps by a specific part of it, namely the one believed to contain common sense. According to his assertion, whenever this small part of the brain is in a certain state, it presents the same signals to the mind, regardless of the states in which other parts of the body may be at that time. This same thesis is transferred to his work "The Passions of the Soul," where, according to Shapiro (2011), this small part of the brain is again identified as the seat of the soul.
In the most influential texts where Descartes makes this assertion, he does not seem to offer any detailed explanation as to why he holds this position (Shapiro, 2011). Nevertheless, Descartes justifies his assertion that the pineal gland is the seat of the soul in two specific contexts. First, he presents a relatively detailed view of the pineal gland in a series of letters addressed to Meyssonnier and Mersenne, starting in 1640 and continuing into 1641. This perspective is later elaborated upon but remains largely unchanged in "The Passions of the Soul." According to Shapiro (2011), the justifications Descartes offers for his assertion are divided into two general categories: anatomical and analogical.
The Anatomical Arguments
The reasons that led him to consider the pineal gland as the control center of the body, as well as the dwelling place of the sensorium commune and the seat of the soul, are undoubtedly based on purely anatomical aspects, as can be inferred from some letters sent by the philosopher during the year 1640, perhaps influenced by the remarkable advance in the knowledge of the nervous system experienced at the time (López, Alamo & García, 2010). According to Shapiro (2011), due to the anatomical location of the pineal gland, it is argued that it is the only structure that allows the soul to think in the way it does while being united to the body.
There are two basic kinds of anatomical arguments (Shapiro, 2011). First, Descartes argued that the anatomical position of the pineal gland uniquely fits the limitations of our sensory perception. Once Descartes announced that the function of the conarium was to be the principal seat of the soul and the place where all thoughts are formed, he proceeded to reason that, since we never have more than one thought at a time, it must be the case that the two impressions from our double sensory organs are joined somewhere in the body; that part should not be double if it is to present a single impression to the mind, from which a single idea can be conceived. Then, according to Shapiro (2011), Descartes found that there is no part of the brain, except the pineal gland, that is not double.
Secondly, the mathematical reasons that led to this choice must be considered, since Descartes chose an organ located precisely at the geometric center of the brain (López, Alamo & García, 2010). Descartes argued that the protected position of the gland within the brain was especially suited to receive ideas and, therefore, conducive to a good and subtle mind (Shapiro, 2011). Although this point may seem entirely different, it turns out that, also for this reason, the pineal gland should be considered the seat of common sense. Thus, as it is the seat of common sense, the pineal gland should also be the seat of the soul. Consequently, according to Shapiro (2011), both types of anatomical arguments aim to demonstrate that the pineal gland is the seat of common sense and, from there, conclude that it is the seat of the soul.
For Descartes, the only alternative to the soul not being connected to any solid part of the body was that, instead, it was connected to the animal spirits found in various cavities, but that idea is patently absurd (Shapiro, 2011). Similarly, according to Shapiro (2011), it was much easier to understand how images from our double sensory organs combined into one within the pineal gland than to understand how they would do so in the cavities.
The Analogical Argument
Apart from the previously mentioned anatomical considerations, Descartes also presents an analogical argument on one occasion to justify his assertion that the pineal gland is indeed the seat of the soul (Shapiro, 2011). In the letter addressed to Mersenne on July 30, 1640, he argued that, since the human soul does not have a double nature but is unique and indivisible, the part of the body to which it is most closely attached must also possess that same characteristic of uniqueness and not be divided into two similar parts. According to his analysis, there is no part of the brain that meets these conditions except the pineal gland (Shapiro, 2011). Moreover, in accordance with López, Rubio, Molina & Alamo (2012), its central location would allow for the integration process of perceptions and sensations from duplicated organs.
Subsequently, he proceeds to dismiss other possible parts of the body as candidates for being the main seat of the soul based on the previously mentioned reasons (Shapiro, 2011). He points out that, although the cerebellum is superficially one and is named as such, even its processus veriformis can be divided into two parts; similarly, the spinal cord can be divided into four parts. Thus, Descartes argues that the pineal gland, being unique and undivided, is particularly suitable for being united with the soul. Since the pineal gland is not divided and is therefore a single entity, and considering that the soul is also one and indivisible, both form a unit. According to Shapiro (2011), it is precisely the analogy in their structures that allows for this union.
References
Gallo, L. E. (2005). De las cosas que pueden ponerse en duda en Descartes: el cuerpo. EFDEPORTES. Recuperado 20 de diciembre de 2021, de https://www.efdeportes.com/efd87/cuerpo.htm
Lokhorst, G. (2013). Descartes and the Pineal Gland. Stanford Encyclopedia of Philosophy. Recuperado 20 de diciembre de 2021, de https://plato.stanford.edu/entries/pineal-gland/#TreaMan
López, F., Alamo, C., & García, P. (2010). La neurofisiología cartesiana: entre los spiritus animalis y el conarium. Medigraphic. Recuperado 20 de diciembre de 2021, de https://www.medigraphic.com/pdfs/arcneu/ane-2010/ane103i.pdf
López, F., Rubio, G., Molina, J. D., & Alamo, C. (2012). The pineal gland as physical tool of the soul faculties: A persistent historical connection. ScienceDirect. Recuperado 20 de diciembre de 2021, de https://www.sciencedirect.com/science/article/pii/S2173580812000429?token=8129F40DB85AB90C253EFF31D92C751CA572A054F1B0562F61E2D42016613BDA86E134C3FA258B088981ACCA2CC14BDD&originRegion=us-east-1&originCreation=20211220191640
Shapiro, L. (2011). Descartes’s Pineal Gland Reconsidered. Midwest Studies in Philosophy. Recuperado 20 de diciembre de 2021, de https://lisacshapiro.files.wordpress.com/2012/04/shapiro-pinealgland.pdf
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