Descartes certainty argument for dualism is invalid by the fact that the property of certain existence is not a property of the thing itself, but only a property of the observer. Hence, the indiscernibility of identicals does not apply to certain existence and therefore does not necessitate discernibility.

Descartes reasoning for arriving at his certainty argument follows from a series of mental exercises he deemed “meditations”. The exercises begin epistemologically, with Descartes seeking the foundations of true knowledge through a method of rigourous doubt. He calls into question everything he has ever believed to be true only to later discover he had somehow been deceived. From this, Descartes wonders how he can be certain of anything and fashions a thought experiment that he purports to be a solid litmus test for absolute certainty.

It is not impossible, he reasons, that he could be possessed by an evil demon dedicated to deceiving him. This demon could construct his entire reality. This is a serious problem for Descartes as it illustrates a possible situation in which almost everything he thinks he knows could be a mere illusion. He cannot be sure that he isn’t possessed by an evil demon, and so he cannot be certain of almost anything at all. Descartes perceptual understanding, he reasons, is uncertain since the demon could be in control of his percepts. He may not have a body but only be led into believing he does. Similarily, Descartes cannot be sure that the physical world exists either; however, would that not also imply that he doesn’t exist?

Descartes arrives at the position that in order to be deceived by an evil demon, he must be. If Descartes did not exist, then the evil demon would have nothing to be deceiving. This is where Descartes arrives at the conclusion that he is absolutely certain of his existence as a thinking thing (for it passes the rigorous test of absolute certainty), but not certain of his existence as a bodily thing (as it does not pass the test).

Here, Descartes reasons there is a property that differentiates his mind from his body, namely, certainty of existence. Since Descartes is certain that his mind exists, but not certain that his body exists, it follows, by the indiscernibility of identicals, that the mind is completely distinct from the body.

However, this reasoning is invalid.

The first premise is convincing. I am certain that I am thinking or experiencing. Whether what I think is true or not is besides the point because I am at least certain that I am thinking or experiencing something, illusion or not. To think or experience presupposes a mind to do as such; hence, my mind must exist. This cannot be escaped. One might attempt an argument that I am simply seeing the world through the mind of another as a trapped observer but, Descartes would agree, I would still be the receptacle to some experience: I would still be a thinking thing. Hence the first premise is sound.

The second premise can be granted for the sake of argument. I’ll concede that it is within the realm of possibility that I could be possessed by an “evil demon” of sorts. After all, my perceptual understanding of the world is surely flawed so I can’t rely on my senses for absolute certainty. One could argue that illusionary thinking, such as that which occurs in dreams or out of the corner of one’s eye is a transitory and ultimately inconsequential state that has no serious bearing on my understanding of reality. Further, it seems that there is always a sufficient explanation for the illusions of the senses. When I see what looks like a figure out of the corner of my eye and turn to see only a tree in the wind, it seems most likely that my mind is on high alert for predators and turns my head instinctively out of self-protection. My understanding of the world, in fact, mostly relies on concepts that I have formed from my perceptual experiences. To throw away the entirety of perceptual knowledge and the world on the basis of a couple insignificant errors of the mind and a thought experiment for which there is no argument for it’s existence is a serious departure from the faculties of reason for the sake of radically doubting knowledge itself. I would be more inclined to agree with this type of argument against Descartes’s second premise than any against the first, but it illustrates a smaller error in Descartes reasoning and so, for the sake of finding Descartes major error, I will consider this argument outside of this scope and grant Descartes his second premise as a sound one.

The leap from premises to the conclusion of this argument is where Descartes makes his major mistake. Though Descartes can claim certainty over his mind and not over his body, that does not show that his mind is distinct from his body. The exact issue is that certainty of existence is not a property describing an object but only a property of the observer. Descartes can be certain that his mind exists and uncertain that his body exists, but that is a function of his mind and not a description of his attributes. It is still plausible that his body exists and encompasses his mind and his certainty would not make a difference.

One might argue that it cannot be the case that one is both certain and uncertain of something’s existence as that is a contradiction. If someone is certain of something’s existence and uncertain of another thing’s existence, that seems to imply that these are different things providing the agent is rational enough not to purposefully contradict himself.

This line of thinking is the same reasonable process that motivated Descartes and it is likely usually the case that one cannot be both certain and uncertain about something’s existence. The condition in which this would not be the case, however, is when one thing has multiple names.

I might have a paintbrush. I am certain that this is my paintbrush as I use it all the time after buying it from a man at the old paintbrush store. Since this man took my cash and did not chase after me when I left with the paintbrush, Let’s say that this is my paintbrush. However, unbeknownst to me and the old man, this paintbrush is actually the same one used by Michelangelo to paint the ceiling of the Sistine Chapel. I might have a friend inspect the paintbrush and exclaim that this is the case, but I might not be convinced that Michelangelo was a real historical figure. I might say that he is a popular mythological figure and that there isn’t any convincing evidence that he ever existed. After all, I have never seen him or a photograph of him. The Sistine Chapel may as well have been painted by some talented priest, and the statue of David was just a piece stolen from Donatello. My friend might ask, bewildered, how I can know that this isn’t the paintbrush used by Michelangelo to paint the ceiling of the Sistine Chapel. While I could claim that I am certain of my paintbrush and uncertain of Michelangelo’s paintbrush hence they are not identical, I am clearly mistaken in this case that my certainty has any bearing on what is identical. Why? Because my certainty of the existence of Michelangelo’s paintbrush does not describe Michelangelo’s paintbrush. If it did, I would certainly have a case on the basis of the indiscernibilty of identicals, but the property of my certainty applies only to my beliefs. As it turns out, the thing which I am certain of and the thing which I am not certain of have exactly the same properties which would lead to the opposite conclusion in Descartes case.

One with an inclination towards post-modernist philosophy might push back on this claim and ask what isn’t assigned to an object as the property of our minds? Can we claim anything to be a property of a thing itself and not simply the product of our minds? If this is true, then certainty of existence is just as valid a property of an object as the concept of divisible by two. Why then should the indiscernibility of identicals allow for fractional equivalence and allow for that which differs by certainty?

These lines of thinking are perhaps the product of the failure of philosophers to define the strict relationship between exact language and reality. It is also the failure of the post-modernist philosophy to realize that the claim that we cannot make an objective claim about reality is, in fact, an objective claim about reality. Nevertheless, to answer what isn’t assigned to an object as the property of our minds, we must have some foundation for epistemology, which the post-modernist skeptic likely intentionally lacks: We must be able to define things as they are in reality beyond some agreeable threshold.

The post-modernist type of thinker might declare that the non-existence of flat surfaces illustrates that our designation of “flat” is socially construed and therefore has no bearing on reality. While the first part is true to a certain extent, it does in fact make a metaphysical claim about reality. Some surfaces are very close to being flat. These surfaces that are “sufficiently close to being flat” are near enough to being flat to warrant a description as such in order to approach a model of reality. If I were to point to two stones: one smooth, and one very sharp, and ask the post-modernist to pick the flat stone, he will very well know what I am talking about and understand that my language has some connection to reality and that the “flat” stone is not a pure social construction. This connection to reality, in fact, only grows stronger the simpler our object is. Mathematical objects, like flat surfaces, might not exist in the way that you can point to the numbers that are divisible by two, but you will have no issue seeing how six oranges are divided evenly enough among three friends, how a building is erected at a ninety degree angle for structural integrity, or how x and y are identical if and only if every predicate possessed by x is also possessed by y and vice versa. So, the social construction theory of our assignment of properties onto reality can only be defended for qualitative properties and, even then, the post-modernist philosopher’s understanding betrays his mystical ideas.

Descartes might still disagree and argue that his assignment of certainty is an objective property that applies to his mind and not his body. I have accepted his premises as maximally true so, if I were to begin describing my body in absolute truth, at some point, I will claim that it is something of which I am uncertain of it’s existence. The same goes for the mind, albeit, much more concise: I will say that it is a thinking thing and that I am certain of it’s existence. These statements are all truths and so how can we accurately describe these things with two distinct and maximally true properties and claim they are one in the same?

This counterargument is much more well-formulated; however, it can be dealt with by a strict and effective definition of what exactly is meant by the properties of an object and differentiating between what I will call positive properties and neutral properties. This will all be reasoned through the constraints of Descartes demon thought experiment. As we will see, neutral properties like Descartes claims about uncertainty, are not properties at all.

In Descartes demon thought experiment, a positive property is that which can known only by the prefix “I experience”. For example, If Descartes were in my living room he would ultimately agree with the claim that he experiences my cat as being orange. Of course, my cat might actually be purple or non-existent and he is actually being deceived by the demon; nevertheless, the experience of my orange cat is still a part of his perceptual understanding and he can still claim certainty of it strictly as a part of experience.

Contrast this with the putative neutral properties. Neutral properties describe truths without describing the object. These can be thought of as a binary choice of which the observer is uncertain. Descartes can look at my cat and experience orange (positive property) but he will also notice that my cat is either a cat or not a cat (negative property). That is certainly a true statement about my cat, but notice that it doesn’t actually claim anything about my cat but only about the nature of reality, namely, the law of the excluded middle. In fact, these neutral properties are a statement that “either something has a property or does not have a property” but that is not a property in itself. Descartes is claiming “either my body exists or it does not exist, I cannot be certain”. These statements are about properties but they do not collapse the binary choice into an actual property. To make a claim about property either requires a positive claim (I experience an orange cat) or a negative claim (I do not experience a purple cat) a neutral claim only states that such properties exist but certainty is required to assign a property; therefore, Descartes certainty of his mind and uncertainty about his body, even as sound premises, do not logically entail the disjunct between body and mind.