This means, it is necessary to direct your attention to the motive of the one who acts (=judge) in order to know the original meaning of the conduct of judging by right and wrong.
So, what is the motive/purpose for the one who judges, when he/she judges (classifies) something by good and evil?
It varies, of course.
There could be an infinite variety of motives for the house wife who divides the onions. There could also be an infinite variety of motives for the one who divides something by right and wrong.
However, you may say, "The purpose of the house wife who groups the onions is usually for the purpose of cooking." Similar to this, you may also generally explain to a certain extent about the purpose of the one who groups something by the use of good and evil.
In this book I would like to explain mainly about the purposes/effects of good and evil, which are related to the following sentences, which I think are the most general: