This dualism seems so compelling (from a layman’s perspective) that it seems difficult to imagine that Descartes invented or even popularized it. For instance, people kept using words like “soul” to refer to the “mind” part that Descartes referred to.
In Plato's Alcibiades , there is a reasoning that goes like this :
(1) Whatever you use as an insrument is distinct from you. ( For example, you use your pencil when you write; your pencil is distinct from you).
(2) But you use your body as an instrument. ( For example, when you write, you use your hand to hold your pencil).
(3) Therefore, your body is distinct from you. Your body is somehing you use and possess, not something you are, in he sense of being identical to).
(4) Consequently, the ( real) human being is the soul ( that which uses the body).
There is a kind of dualism here, but Plato's dualism may not be ttermed a substance dualism, for he does no attribute a substance status to human body ( nor to maerial objects in general).
Descartes alludes to Plato when he corrects his own dualism saying : " I am not in my body as a pilot in his ship".
You may have a look at : http://www.alevelphilosophy.co.uk/handouts_religion/DescartesIntermingling.pdf
'I am not in my body as a pilot in his ship', derives from and modifies a text not in Plato but Aristotle, to the best of my knowledge. In de Anima, II.1, 413a Aristotle observes:
But it remains unclear whether the soul is the actuality of a body ... or rather is as a sailor of a boat' (de Anima, tr. H. Lawson-Tancred, London: Penguin, 1986: 158).
Descartes removes Aristotle's hesitation by his own emphatic denial of the sailor/ boat analogy.
As for the philosophical origins of dualism, a version of it is clearly present in Plato's Phaedrus, 250c, where he refers to 'this which we carry about with us and call the body, in which we are imprisoned like an oyster in its shell' (H.N. Fowler translation, Loeb, 1947). What is imprisoned is the psuche - soul or mind but no single English word precisely corresponds to it. But psuche certainly contains elements that we would include in later (including Cartesian) senses of 'mind'. In this sense it is at least one early version of dualism. I am not aware of an earlier version. (I.M. Crombie assesses Alcibiades 1 & 2 as 'Quite likely not by Plato' and says of Alcibiades 1, the relevant portion of which is 129e-130c: 'If by Plato, then fairly late', i.e. post-Phaedrus: I.M. Crombie, Plato and His Doctrines, 1, London: Routledge, 1962: 12.)
Unless, that is, we grant some latitude to a saying attributed to Pythagoras by Aristotle in Diels-Kranz, Die Fragmente der Vorsokratiker, 5th ed., 1954, Fr. 196:
... the ringing sound of bronze when struck was, he [Pythagoras] said, the voice of a divine being (daimon) imprisoned in the bronze (The Presocratic Philosophers, 2nd ed., ed. G.S. Kirk et al., Cambridge: CUP, 1983: 236).
If Descartes did not originate mind/ dualism he brought it to the forefront of modern philosophy. In that sense he was its principal 'popularizer'.
Many ancient sages (philosophy was hardly a separate discipline in those early times) speculated or pronounced on the relationship between mind and body. The issue hinges around the notion of an afterlife; if you believe in that, then it is hard to deny dualism because the body is left behind to rot. Most ancient cultures had their Gods, disembodied ghosts and so forth, so dualism came as a natural consequence.
Some, such as Buddhism and Taoism (ca. 500 BC), were more circumspect. They took the view that the world we experience is an illusion, the only true existence is our inner nature.
Siddartha Gautama seems to have encountered almost every view imaginable during his Indian search for enlightenment. However it is hard to argue that reincarnation is not a dualistic concept, which suggests that Buddhism is still essentially dualistic.
One might argue that Taoism gave birth to the first true monism, in which the search for immortality was seen by some writers to mean physical Earthly immortality. However the European alchemists also saw the philosopher's stone in that same light, yet most would have claimed to be dualistic Christians. So such claims of monism are moot.
In Timaeus (ca. 360 BC), Plato's protagonist of the same name proposed three independent levels of being, accepting the physical world but separating out the ever-changing mind from the eternal ideas which preoccupy it. One may draw parallels there to modern information science and the recent appearance of religions which worship information in its own right.
I seem to recall that it was Alexander the Great (ca. 330 BC) who encountered a tribesman who took him to his home. There he had set up a phallus idol, which he worshipped. On being asked why, he stated that he knew of none other who had made him. This provides an early example of the atheistic monism which would come to dominate the intellectual climate of the twentieth century.
However dualism remained the default. The modern challenge has its roots in the 1400s with the Renaissance development of rationalism and humanism. By the time of Descartes these were challenging religious dogma as a basis for belief. He too was a rationalist and his unique contribution was to present rational arguments for dualism and deity, thus to some extent healing the developing rift. So it is perhaps more correct to say that Descartes created the modern rationalisation of that ancient belief, thus helping to maintain its popularity in an age of rationalistic reappraisal.
Reasoning about the basic nature of ideas, things, and truth ultimately began in earnest with the Pre-Socratic philosophers, so he doesn't get credit for inventing dualism. While René Descartes did not invent mind-body duality, he did popularize it, and in fact, many people often use mind-body dualism and Cartesian dualism interchangeably, although technically they are not, the latter being a kind of the former.
Strongly since the middle of the 20th century, certain schools of analytic philosophers have launched strong attacks on Cartesian duality. Two of the most famous are Gilbert Ryle and Daniel Dennett who both view such duality as a category mistake, but many famous philosophers over the centuries have. While Dennett rejects the mental as an illusion in his version of eliminative materialism some reject materialism most notably George Berkeley for his views known now as subjective idealism.