Full Idea
Descartes' Cogito posits as 'true a priori' our belief in the concept of substance, but the idea that when there is a thought there has to be something 'that thinks' is simply a formulation of our grammatical custom that adds a doer to every deed.
Gist of Idea
The Cogito assumes a priori the existence of substance, when actually it is a grammatical custom
Source
comment on René Descartes (Meditations [1641], §2.26) by Friedrich Nietzsche - The Will to Power (notebooks) §484
Book Reference
Nietzsche,Friedrich: 'The Will to Power', ed/tr. Kaufmann,W /Hollingdate,R [Vintage 1968], p.268
A Reaction
This anticipates the sort of thing Ayer and the logical positivists said. It is not clear that Descartes does think the mind is a substance, but this pinpoints a possible presupposition in Descartes.