Cartesian dualism tends to be one of the most troublesome concepts in the history of philosophy. Not because of the intrinsic difficulties that are required to effectively criticize such an outdated notion (there are none), but rather, because of the consequences that are brought about by its widespread belief. Everything, ranging from our morals, to our justice system, is at least partially buttressed by the idea that we are not merely materialistic beings, reduced to atoms and molecules, but that there is something else, some metaphysical entity that supersedes on our physical endeavors, some “ghost in the machine”, so to speak.
Cartesian dualism has been one of the most enduring philosophical responses to the mind body problem. It holds that mental events, in their most fundamental sense, are different than physical ones. The mind (as in the thoughts, emotions, and subjective experiences that we have), is absolutely orthogonal to the physical processes that underpin our existence (your neurons firing, your hormones secreting, etc.), Dualists claim. To further elucidate what this means, I might provide an example.
Suppose I pinch you, and you scream “ouch”, this event can be described through two different avenues. The mental, is the subjective feeling of pain that you had when I pinched you. The physical explanation, on the other hand, relates to the observable physical manifestations that arise from me pinching you, in this case, your sensory neurons firing in your nervous system, along with you jerking and screaming “ouch”, and everything else that surrounds it.

As you can see, the difference between the physical and the mental is that the former is scientifically observable, from a third person perspective, while the latter is inherently a first person phenomenon. While anyone in this planet can assuredly confirm that you expressed pain by simply watching you cry, no other person other than you, can have access to your actual feeling of pain; to your inner world of thoughts, emotions and feelings. No matter how many fMRI machines I build, or how groundbreaking our investigative tools become, I will never be able to determine what it is that you feel when you’re having a sip of coffee, or showering in cold water. All I am able to determine with these inquisitive tools are the patterns that correlate with such self-reported experiences (e.g. high cortisol levels usually correlate with people self –reporting feelings of anxiety, caridovascular exercise and the secretion of endorphins in the brain that comes along with it, is correlated with a self reportedly relaxed state of mind), but we can’t go further than that.

The nature of our mental lives, makes it impossible for us to inquire about the nature of others’ mental experiences, it is a private sphere, sealed off from outer examination. With this said, and holding in mind the fact that according to Dualists, the mind is totally distinct, and unrelated to the body, one stumbles upon some pretty strange conclusions that seem to inevitably follow.

For instance, although the physical and mental are in principle completely unrelated, it is a fact that in your case, the two correlate. When you’re happy (subjective mental state) you usually smile (physical manifestation), when you feel stressed (subjective mental state), your blood pressure goes up (physical manifestation). But remember, that the only mental world to which you have access to ,is your own, hence it would be insensible to assume that everyone’s mental world correlates to their physical body in the same way that it does with yours. If the mind and body are not related at all (as Cartesian dualists hold), then the fact that your mental events correlate with your physical manifestations in the way they do, could just be mere coincidence. In other words, it is not intuitively obvious that the kinds of physical happenings that seem to go along with mental experiences should be the way they are. Why should it be obvious that a sense of sadness be expressed through tears? How do you know for sure that tears usually indicate sadness in other people? For all you know, all you can confidently make conclusions about, are your own mental states. To put it another way, if we were to mathematically plot on a Cartesian plane (so ironic!) the correlation between mental events and physical ones, we would not be able to make any significant assertions about it, mainly because we would only have one point in the plot (our own). It would be irresponsible to blindly assume that everyone's physical states correlate with their mental ones in the same way they do in us (or that they have any mental life at all for that matter).

Following this dangerous logic leaves open possibilities that would seem nothing but ridiculous to any sound person. If the mind and body are after all, separate, and not necessarily correlated, it can be possible that while punching you in the face causes you to subjectively feel pain, punching your friend makes him experience a great deal of orgasmic pleasure. When you are seen laughing powerlessly to some joke you heard, you are subjectively experiencing a funny feeling, a feeling of laughter. But when your friend is seen laughing, he is undergoing the deepest feelings of misery any human being can have. These are the outcomes that one has to maintain as possible if he claims that mind and body are in principle, unrelated.
All of this follows of course, if we are generous enough to grant the possibility that other human beings have a mental life at all. For all we know, they might also scream when “mad”, and their face might still turn red in circumstances in which they would be expected to be angry, but subjectively not be feeling anything at all. If the physical and mental don’t affect each other, and they aren’t intertwined, then these possibilities must necessarily remain open.