Descartes refers to the ‘I’ as “in the strict sense, only a thing that thinks. That is, I am a mind.” He calls the ‘I’ the author of thoughts. The body, on the other hand, is not who the ‘I’ is, but rather a thing which the ‘I’ has—or, at the very least a thing which the mind perceives itself to have. “I am not that structure of limbs which is called a human body.” “I ha(ve) a face, hands, arms, and the whole mechanical structure of limbs which can be seen in a corpse, and which I (call) the body.”
By these definitions the mind and body must be two distinct entities. While the body is something the mind has, the mind, Descartes claims, is a doer of things. “(I am) a thing which thinks, … a thing that doubts, understands, affirms, denies, is willing, is unwilling, and also imagines, and has sensory perceptions.” It then follows that “to be” is “to do”—at least a certain kind of doing.
To doubt, understand, affirm, deny, be willing, or be unwilling one must analyze information. That is, to think is to interact with information. Information must first be received for analysis to occur. Descartes mentions two possible ways to receive information: sense perception & imagination. To sense perceive is to receive information I consider to be real from that which I do not consider to be myself. But, imagining is perceiving what is non-real, that is, to imagine is to take in information which I consider to be unreal from that which I consider to be myself. The doing we call “thinking” is based on either an interaction between a real self and a real non-self or an interaction between a real self and a non-real self. He says, “this puzzling ‘I’ … cannot be pictured in the imagination.” This is because to imagine (or sense-perceive) there must be an interaction with a real self. If ‘I’ were able to imagine a self, it could only be a non-real—as it would be composed of information the real self obtained from a non-real self or a real non-self.

The I cannot be pictured in the imagination.

Descartes’ mind/body dualism is the result of three considerations—namely the epistemic, those of physics, and those of comparison between animals and machines. First he asks what we can know and how much certainty we can claim. The senses deceive. This is because we do not perceive reality itself, but our thoughts regarding reality. “(when I) see men crossing the square … I normally say that I see the men themselves … Yet do I see any more than hats and coats which could conceal automatons? I judge that they are men.” Thus, the senses are filtered through the mind. The ability to discriminate between two things is built up from our past experiences. The senses send a limited range of information—for example, we do not see microwaves, but only visible light waves—our brains compare that information to remembered objects and events, and then tell our minds that we have witnessed a thing which is like a specific set of things in a certain way and unlike other sets of things in certain ways.
If I ask you to think of “apple” without further context, our minds parse through all the things which we have in past labeled “apple”: Granny Smith, Golden Delicious, Honeycrisp, Fuji, McIntosh red, Apple Macintosh, MacBook, iPhone, Steve Jobs, Apple Corps., the Beatles… Your mind accepts that it has perceived these real objects in past and has fit to them the label “apple.” Having no further filtering information, it doesn’t stop on the first one which comes to mind, but skims through all items thus labeled.
Knowledge of the body and its existence relies on imperfect sense perception and the mentioned filtering process. Knowledge of the mental self, however, doesn’t require either. As Descartes says, “It is possible that what I see is not (real) … It is simply not possible that I who am now thinking am not something.” Which is to say, because I am aware of my thinking, ‘I’ at least must exist.
L’homme de René Descartes, et la formation du foetus

L’homme de René Descartes, et la formation du foetus

He also discusses that we can be fooled into believing that dreams are reality. So, we cannot be certain that any non-self thing is indeed real. From both of the above he concludes that the mind exists necessarily, while all else—including the body—exists contingently.
Decartes’ third consideration is as tenuous as his second. It is as follows: ① The bodies of men and animals are analogues. ② The physical principles which govern bodies are the same. ③ No matter their mental abilities, men can make their thoughts understood to others through language. ④ Despite having the organs necessary for utterance, animals don’t demonstrate skillful use of language. ⑤ Animals cannot demonstrate what they are thinking. ⑥ ∴ “They have no intelligence at all.”

^{Kanzi, the bonobo, is famous for his use of language.}

Premise 4 can be refuted on several grounds. First, several animals show understanding and use of language both in the wild as well as under clinical conditions. This includes animals such as the bonobo Kanzi constructing novel sentences, use of language to demonstrate the desire to do something no one ever told him possible (e.g., cooking marshmallows over a fire), as well as complete understanding of grammar (“put the bag in the box” vs. “put the box in the bag”). Beyond animals with even less human-like physiologies are known to use language, or complex communication at the very least. For example, vervet monkeys will signal different calls depending on the predator spotted: leopards, eagles, pythons, or baboons. Beyond the primate realm, crows are able to distinguish human faces and relate information about them to others within their murder. Besides using different forms of language (or communication) non-human animal vocal organs do not allow for the use of human-like language. For example, it is thought that H. sapiens neanderthalensis had the brain structure required for human language, but their hyoid bone was positioned similarly to an infant H. sapiens sapiens making their speech unintelligible (or nearly so) to early humans.

Descartes also states that human-like machines would be unable to fool people into believing that they are humans. This has yet to be proven false, but Alan Turing described the conditions under which he believed it could be done. Modern programmers and artists have so far been frustrated in their attempts to surmount the “uncanny valley”—the unsettling feeling experienced when people “meet” human-like machines.

^{Uncanny valley of the dolls.}

Beyond the objections above listed, thinkers since Descarte have rejected his substance dualism. The sciences depend on externally observable phenomena and the mind cannot be observed objectively. This has led to a split in academia. On one side are biologists, psychologists, and most anthropologists stating that we can only scientifically study the objectively true—the physical brain, behaviours, and cultures, respectively—and thus that which is quantifiable. On the other side sociologists and some anthropologists, many of whom reject the concept of objective truth, depend on subjective and qualitative data.
Psychologists focus on behaviour because it is both highly observable—by external agents—and highly manipulable—again, by external agents. It is the most cost-effective and time-effective method for obtaining “mental” data. Moreover, because scientists cannot objectively observe the “mental” or “spiritual”, the terms are meaningless. This leads to a substance monism in which everything has a physical basis.

Attributions

^{Images:
"Mind/body dualism" via Max Pixel, CC0: public domain.
"Homunculus of the mind" via Soen's Cognitive album cover, modified by article author.
"L’homme de René Descartes, et la formation du foetus" via Wikimedia Commons, public domain due to age.
"Kanzi" via Wikimedia Commons (William H. Calvin, PhD), CC Share Alike 4.0 International license.
"Uncanny valley of the dolls." via PxHere, CC0: public domain.

Quotations:
Second Meditation & Sixth Meditation from Meditations on First Philosophy in which the existence of God and the immortality of the soul are demonstrated by René Descartes, 1641
Part Five Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences by René Descartes, 1637}

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