Full Idea
From the fact that I am thinking it follows that I exist, since that which thinks is not nothing. But when he adds 'that is, I am a mind, or intelligence, or intellect', a doubt arises. ..You might as well say 'I am walking, therefore I am a walk'.
Gist of Idea
That I perform an activity (thinking) doesn't prove what type of thing I am
Source
comment on René Descartes (Meditations [1641], §2.26) by Thomas Hobbes - Objections to 'Meditations' (Third) 172
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.70