There are two ways to present Descartes’ thoughts on mind, body and the interrelationship between these two and both are entirely inappropriate. Let me say a few things about his methodology to make clear what I mean: Descartes decided to set aside everything that wasn’t obviously and entirely true, just as if it had been proven false. This has astonishing consequences: Everything our senses tell us has to be assumed possibly wrong – in fact, the assumption that we have senses goes one step too far.

Start with the simplest possible assumption set, allow the addition of obvious-in-hindsight ideas exclusively and if you manage not to stray from this principle throughout your meditations, what you end up with is a philosophical framework that is both too deceptively simple to be part of an impressive, serious-sounding philosophical essay and too deep to let itself be obfuscated by a bunch of unnecessary technical terms Descartes himself, sitting in the large bread oven where he used to spend much of his time thinking and meditating, would have laughed at.

Starting with a reconstruction of Descartes’ train of thoughts, we will see where it took Descartes, what he definitely got right and what we, 400 years later, can say about the rest of his conclusions.

Descartes first insight is simple enough that most philosophers of Descartes’ time took it for granted as an unwritten axiom: There is something I know for certain. Nowadays the most popular of Descartes’ utterings, “Cogito ergo sum” — I think, therefore I am – expresses the idea that, even if someone were to deceive us in everything else, we can never be deceived about the fact that we exist and that we exist as a thinking thing – something that is able to be deceived. In as far as the distinctness of mind and body is concerned, this first insight leads us to the conclusion that there is at least one substance, namely the substance of the mind.

None of the attributes that belong to our body, be it touch, sight, hearing, taste or smell and the fact that we can move and be moved, do essentially belong to ourselves since we could be deceived regarding their existence, as is, for instance, the case when we sleep. We conclude that we are not identical to our bodies, therefore – if bodies exist – mind and body need to be distinct.

In order to claim that mind and body are two separate kinds of entities, Descartes needs to provide proof – or, at least, strong evidence – for the fact that bodies exist. It turns out to be helpful to consider the way we think about our environment, since there are two distinct ways we can think about the outside world: Imagination and intellection. Imagination is what makes us “see” the world in our head when we think of a clear, blue sky – a perception everyone of us has had – in contrast to what goes on in our head when we think of how it might feel for a bat to find its way through a cave by means of echolocation. Intellection describes our ability to actually think of the latter – to have thoughts independent of whether we may ever have any experiences similar to these thoughts in the real world or not.

Descartes notices that it is not hard to imagine our existence without the ability of imagination (although we cannot imagine imagination to exist without a thinking substrate). Since imagination exists and feels like the immediate application of our cognitive abilities to the world which is present around us, we should assign a high probability to the idea that the world around us does in fact exist, both distinct and connected to our mind.

Without knowledge about the existence or non-existence of an all-powerful creator, we can’t say much more about the distinct existence of our body. If we accept the premise that an omnipotent non-deceiving god exists – a premise Descartes builds on his ontological argument – we can derive quite a few more conclusions about our environment.

We can’t deny that we do have very distinct ideas on what constitutes a body and what constitutes a mind. Since an omnipotent god could create everything that we can imagine as being distinct from one another in way that makes them actually be separate from one another, and since God does not deceive in a way that we can’t correct for, there has to be some truth in everything nature teaches us, therefore we have to assume that mind and body are separate from one another.

If we allow ourselves to assume that there is a passive faculty of perception, there needs to be an active faculty that generates the perceptions that reach our senses. This faculty needs to persist in a substance, just as our mind does, but this substance does not appear to be the substance of the mind: Perceptions that seem to come from the outside world don’t appear to presuppose thought and, as is the case with pain, they can be contrary to our own will. Thus it is likely that corporeal objects exist, even if in a different form from the one that appears to our flawed senses.

There is one further single line of reasoning that Descartes would have felt to be sufficient if it were not for the plethora of other arguments he could come up with: Bodies are always divisible, the mind is indivisible, therefore the two can’t be of the same substance.

The first thing anyone who reads a few pages of Descartes’ meditations and can’t avoid forming an opinion on these topics must notice is, that, keeping in mind that Descartes started from scratch, he got surprisingly far.

Descartes opinion on the existence of intellection and imagination – both exist, although one might have been sufficient without an external, physical world, therefore we should consider the existence of an external world likely instead of making up some more contrived reason for the existence of our imagination – sounds like an application of Occam’s razor, one of the most basic heuristics of modern science, and thus makes a lot of sense.

To my mind, Descartes fails to appropriately consider how the interface between the two substances would look like if the substance of the mind and the substance of the body were essentially distinct: How do they interact with each other? It seems quite obvious that the material world does influence our mental world, and Descartes doesn’t ignore this fact. He himself talks about an active and a passive faculty interacting with each other, but glances over the details, which is probably in part attributable to the fact that the neuroscientific knowledge of the 17th century was quite limited.

Judging from his legacy in math and philosophy and from the fact, that he was smart enough to write down the results of his meditations for posterity, Descartes appears to have been a pretty sharp guy. Nonetheless he came to the conclusion that his ontological argument was powerful enough to make its conclusion – an omniscient, omnipotent, beneficent creator exists – part of his further philosophical essays, without much written consideration on where his meditations might have taken him if such a creator did not exist. We, who are certainly less gifted than Descartes was, might want to pay attention to ideas that cut off certain pathways of thought when thinking about matters of importance.