Reason, Sense, Intellect
noun
If you have reason in your nature, you have reason, sense and intellect. This is "reason" as in logical thinking.
りせい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
お酒を飲みすぎて、完全に理性を失ってしまっていました。
I had too much drink and I lost all sense of reason.
できるものなら、いつだって理性的でいたいと思っていますよ。
If possible, I always want to be rational.
ダメだと分かっていたが、理性を保てなかったんだ。
I understood it was hopeless, but I couldn’t hold on to reason.