Full Idea
If anyone asks me: What is the constitution of a thing that thinks? I do not know how to answer a priori, because the answer ought to be synthetic (for an analytic answer explains thinking, but gives no cognition of that on which thinking rests).
Gist of Idea
We cannot give any information a priori about the nature of the 'thing that thinks'
Source
comment on René Descartes (Meditations [1641], §2.26) by Immanuel Kant - Critique of Pure Reason A398
Book Reference
Kant,Immanuel: 'Critique of Pure Reason', ed/tr. Guyer,P /Wood,A W [CUO 1998], p.440
A Reaction
This has always seemed a problem with Descartes' very thin account of his 'res cogitans', but then what exactly does Kant want to know? Is it a metaphysical disaster if we think of the self as having no more identity than a geometrical point?