I really tried finding the original passage where Descartes lays out the following argument:
Descartes himself anticipated an objection like this and argued that dependence does not strongly support identity. He illustrates by means of the following example: a virtuoso violinist cannot manifest his or her ability if given an instrument in deplorable or broken condition. The manifestation of the musician's ability is thus dependent upon being able to use a well-tuned instrument in proper working order. But from the fact that the exhibition of the maestro's skill is impossible without a functioning instrument, it hardly follows that being skilled at playing the violin amounts to no more than possessing such an instrument. Similarly, the interactionist can claim that the mind uses the brain to manifest it's abilities in the public realm. If, like the violin, the brain is in a severely diseased or injurious state, the mind cannot demonstrate its abilities; they of necessity remain private and unrevealed. However, for all we know, the mind still has its full range of abilities, but is hindered in its capacity to express them.
If you could give me the exact quote or a reference where to find it in his works?