The following is the beginning of my translation of the Ralbag's M'komot. Enjoy!
The first Makom [Topos]:
The Torah will cite a particular in place of the general. There are two types:
The first type:
From the particular mentioned in a mitzva the more general category is learned.
For example, we are commanded in the Torah (D'varim 22:10) that we may not plow with an ox and a donkey together. The Torah cites "ox" and "donkey" in place of any two species - one being tamei and the other tahor; "plowing" is cited in place of any craft that would employ a tahor animal with one that is tamei, as is explained in the eight chapter of K'laim and other places throughout the Talmud.
It will be clear when to learn from the particular the general and when not, from the subject matter of the mitzva and the words that are employed - as will become clear to you from our discussion, when we utilize this makom. For example, the subject matter of this mitzva demands this law apply in the case of other animals - meaning to say, that they should not be joined in one craft, one being tamei and the other tahor, as will be explained there, with God's decree.