Full Idea
It may be that the thing that thinks is the subject to which mind, reason or intellect belong; and this subject may thus be something corporeal.
Clarification
'Corporeal' means physical.
Gist of Idea
The 'thinking thing' may be the physical basis of the mind
Source
comment on René Descartes (Meditations [1641], §2.27) by Thomas Hobbes - Objections to 'Meditations' (Third) 173
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.70
A Reaction
Of course, Descartes goes on to reject this view. Presumably he is suggesting that mind etc. might be properties of something corporeal, rather than being identical with it. Descartes was well aware of materialism in Hobbes and Gassendi.