Why, then, did a mind-body problem ever arise? What made it ever seem like a problem worth accepting, rather than a reductio of one’s philosophical views? What benefits did Descartes think outweighed its intolerable difficulties? How in the world did he get anyone to agree with him?
The answer, I think, cannot come from medieval philosophy of psychology. As far as the Middle Ages were concerned, the mind-body problem was a non-starter, the dust having settled since Plato’s flirtation with it in Antiquity. Medieval anticipations of practically everything else can be found, but not the mind-body problem. The answer must lie elsewhere.
Let me suggest by way of conclusion that the culprit might be the distinction between primary and secondary qualities. The success of the New Science made it seem plausible that the only ‘real’ properties in the world were fundamentally quantitative: the primary qualities of size, shape, location, speed, direction. But that left secondary qualities with nowhere to go, so to speak. They had to migrate from the external world (where they had happily been since Antiquity) to the only place left that still seemed inexplicable in quantitative terms, namely the mind.
Peter King, Why Isn't the Mind-Body Problem Medieval? (PDF Format)
High-falutin' but worth a read. A more accessible treatment of the topic is available in Alfred Freddoso's essay Good News, Your Soul Hasn't Died Quite Yet