I might be mistaken, but the translation for「理性」was "reasoning", but on page 4 of Chapter 14, it's being translated as "resolve", and I think "reasoning" is more accurate. At least the definition in most dictionaries lean more to the former. And it would make sense to me, since Ryou seems more concerned with being able to think properly (i.e. think reasonably), though "determination" doesn't seem too far off in context.
I also want to point out what seems to be clever wordplay at work having Ryou use the word 「理性」, because「理」can mean "truth" (as in 真理)and「性」is of course "biological sex" or "gender" (as is 女性・男性). So when Ryou say「俺の理性が。。。」, it's kinda like them saying "Oh no, my true gender..." Well, only if you were to dissect the kanji in such a way.
This is why I love Japanese.