Reason
Sense, Intellect
noun
りせい
理由 （りゆう） Reason, Motive, Excuse
理性 （りせい） Reason, Sense, Intellect
故 （ゆえ） Reason, Circumstance, Cause
訳 （わけ） Reason
理性 is a broad word with several meanings because it uses the nature 性 kanji.
He kept saying rease- りせい instead of Reason.