reason

noun

the capacity for rational thought or inference or discrimination
“Some people think that humans are endowed with reason and capable of distinguishing good from evil.”
understanding
a fact that logically justifies some premise or conclusion
“There is reason to believe they are lying.”
an explanation of the cause of some phenomenon
“the reason a steady state was never reached was that the back pressure built up too slowly”
a justification for something existing or happening

cause
a rational motive for a belief or action
“the reason that war was declared”
ground
the state of having good sense and sound judgment

reasonableness

verb