Full Idea
First, I maintain that the mind, which we often call the intellect, the seat of guidance and control of life, is part of a man, no less than hand or foot or eyes are parts of a whole living creature.
Gist of Idea
The mind is a part of a man, just like a hand or an eye
Source
Lucretius (On the Nature of the Universe [c.60 BCE], III.95)
Book Reference
Lucretius: 'On the Nature of the Universe', ed/tr. Latham,Ronald [Penguin 1951], p.99
A Reaction
Presumably Lucretius asserts this because some people were denying it. Sounds like common sense to me. The only reason I can see for anyone denying what he says is if they are desperate to survive death.