Justification or motive (6)
I believe the answer is:
reason
I believe this is a double definition.
'justification' is the first definition.
(I know that reason is a type of justification)
'motive' is the second definition.
(I know that reason is a type of rational motive)
(Other definitions for reason that I've seen before include "Explanation for some event" , "Justification; sanity" , "Explanation; logic" , "Logic and rationality" , "Deduce" .)