Short explanation: the person that knows why a thing must be done is generally the person who decides what must be done. Application to rationality: instrumental rationality is a method that serves goals. The part that values and the part that implements are distinct. (Also, you can see the separation of terminal and instrumental values.)

It's fairly benign, but looks a little archaic -- not so archaic that it'd have to be medieval, though. The rest of the phrasing is fairly modern, or I'd probably have assumed it was a quote from anywhere from the Enlightenment up to the Edwardian period. It has the ring of something a Victorian aphorist might say.