Logically sound (8)
(Other definitions for reasoned that I've seen before include "Spoke logically, thought and drew conclusions" , "Argued or deduced" , "Based on logic" , "Debated in calm and logical way" , "Argued, concluded" .)
'logically sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for valid that I've seen before include "Legally accepted or logical" , "Reasonable" , "Still legally acceptable" , "Legitimate" , "Legally acceptable, unexpired" .)