Perceive or notice (6)
I believe the answer is:
detect
I believe this is a double definition.
'perceive' is the first definition.
(I know that detect is a more specific form of sight)
'notice' is the second definition.
(I know that notice can be written as detect)
(Other definitions for detect that I've seen before include "Notice, sense" , "Find out like Sherlock" , "Discover existence of" , "Perceive - discover" , "Do what Morse did" .)