Full Idea
In one of his last letters Descartes describes incorporeal substances as 'powers or forces'.
Gist of Idea
Incorporeal substances are powers or forces
Source
report of René Descartes (Two letters on mind [1649], Feb 1649) by Robert Pasnau - Metaphysical Themes 1274-1671 08.4
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.157
A Reaction
Only a glimmer, but I really like this idea. (Ellis flirts with it somewhere). Minds are deeply and intrinsically active things. Try ceasing to think for five minutes. Apparently 12th century Cistercian authors were keen on the idea.