Reason is an ancient phrase now used as a term to define the cause of events and any formal system of thinking using cognitive abilities to conceive, judge, deduce or imply such causes.
The word Reason is derived from an ancient Gaelic word reasun meaning literally “what is your frame of mind, mood, humor today?” from re meaning “time period”, as meaning “as / in” and sun / sund meaning “frame of mind, mood, humor”.