Mix active and passive voice in the thesis

This is a tough question. I have faced it myself.

You are caught in a tug-of-war between differing schools of thought in the matter. Unfortunately, it is likely that at least one professor on your dissertational committee feels strongly in the matter. Fortunately, if you just accept your advisor's advice in the matter, then the professor in question will almost certainly take the argument to your advisor and pass your thesis. What you cannot afford to do is to buck your advisor.

Abstracts regrettably are often badly written. The most standard style, recommended by the best editors of U.S. journals, seems in my experience to be not "In this paper I prove that the ..." but merely "It is proved that the ..." or, better, "That the ... is proved." This is not so much English as Abstractese, of course, but Abstractese actually serves a valid purpose. It is a dialect worth mastering.

Regarding the active and passive voices, you have never mentioned your major, but there simply exists no consensus for technical, mathematical and philosophical papers, which are not literature and do not benefit as much as literature does from the active voice. Still, English does like the active voice as a rule. My own view, after many years of trying both styles, is that the passive voice really is better for the kind of paper or thesis in question—but as I said, my view counts for nothing here. Go with your advisor's preference. Even if your advisor's preference disagrees with your own, you will learn something by trying it.

Incidentally, mixing voices in the specific manner you suggest is not always easy but can be done to pleasing effect. Of course you should speak of the work of others in the active voice, as you suggest. The passive voice is usually silly to use in that context.

Good question.