At "Analysis" there's a very interesting post on Descartes and the mind-body distinction. Descartes more or less argues:
1.I understand the mind to be indivisible by its very nature.
2.I understand body to be divisible by its very nature.
3.Therefore, the mind is completely different from the body.
The author suggests that the hidden premise here is:
(HP) If the mind and body have different properties, then they are completely different.
This is, of course, highly implausible. But there are other premises that would get you from (1) and (2) to (3), and I suspect that Descartes's actual assumption here is:
(D/I) What is indivisible by its very nature is completely different from what is divisible by its very nature.
Which is actually very plausible. This is similar to the suggested revision farther down in the post, but wouldn't require appealing to indiscernibility of identicals but rather to the principle of noncontradiction.