Meaning of "a wisp in the ether"?

What is the meaning of the phrase a wisp in the ether? Here is the whole paragraph:

But Descartes believed Galen's explanation for humans as similar to sheep could not be reconciled with the notion that humans were created in God's image. He thought the mind stood separate from the physical body as a wisp in the ether that could act on the body.

I don't know how widely used it is as a phrase (I don't think I've heard or seen it before), but it is fairly clear what it means.

Wisp is a very slight trace (wisp of smoke), or minute quantity (wisp of hair), or otherwise almost invisible (will-o'-wisp).

Ether would probably be better written æther and refers not to the family of chemical compounds but to the all encompassing substance that the Michelson-Morley experiment proved did not exist (some hundreds of years after Descartes time, of course).

Therefore, 'the mind stood separate from the physical body as a wisp in the æether' refers to the mind being like a will-o'-wisp that cannot really be seen, but can influence the body. Mind is qualitatively different from the body, yet undetectable except through its effect on the body.