Reason
Sense, Intellect
noun
If you have reason in your nature, you have reason, sense and intellect.
This is "reason" as in logical thinking.
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.
ビエトは、今時は、ヤクザもぼう力にたよるんじゃなく、理性にうったえなくっちゃダメだって言ってたよ。
Viet said that nowadays even yakuza have to appeal to intellect instead of resorting to violence.
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.