Full Idea
Consciousness is an operation of the understanding of its own kind, and cannot be logically defined.
Gist of Idea
Consciousness is an indefinable and unique operation
Source
Thomas Reid (Essays on Intellectual Powers 6: Judgement [1785], 5)
Book Reference
Reid,Thomas: 'Inquiry and Essays', ed/tr. Beanblossom /K.Lehrer [Hackett 1983], p.267
A Reaction
It is interesting that has tried to define consciousness, rather than just assuming it. I note that he calls consciousness an 'operation', rather than an entity. Good.