Full Idea
That it is the inner air that perceives, as being a fragment of the god, is shown by the fact that often when our minds are preoccupied with other matters we fail to see or hear.
Gist of Idea
Perception must be an internal matter, because we can fail to perceive when we are preoccupied
Source
report of Diogenes (Apoll) (fragments/reports [c.440 BCE], A19) by Theophrastus - On the Senses 42
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.200