St. Thomas Aquinas scripsit: “Ea vero quae dicuntur ad aliquid, significant secundum propriam rationem solum respectum ad aliud. Qui quidem respectus aliquando est in ipsa natura rerum; utpote…”
What does “qui” mean here? I think this is a common use in medieval works but I’m not sure how it is working here and I don’t know what I’d say it means in English.
“…Indeed sometimes a relation is in the nature itself of the thing…”
A relative pronoun can begin a sentence in Latin. Then it’s often natural to translate it as demonstrative or personal pronoun. This usage connects the sentence more closely to the previous one. There is in actual fact nothing specifically mediaeval about this: it’s current in classical Latin, as well.
It’s tantamount to “Et hic.” Which [= And this] is how we sometimes use relative pronouns in English too. “And indeed this respectus …” (lit. which respectus indeed …).
The period (full stop) before it makes it strictly speaking ungrammatical. A dash might serve equally well.
in ipsa natura rerum is “in the very nature of things”