The word philosophy means the love of wisdom, but what philosophers really love is reasoning. They formulate theories and marshal reasons to support them, they consider objections and try to meet these, they construct arguments against other views. Even philosophers who proclaim the limitations of reason--the Greek skeptics, David Hume, doubters of the objectivity of science--all adduce reasons for their views and present difficulties for opposing ones. Proclamations or aphorisms are not considered philosophy unless they also enshrine and delineate reasoning.
Which should be easy enough, given that all proclamations or aphorisms, of whatever kind, "enshrine and delineate reasoning"; it is what makes them intelligible in the first place. This whole passage, in fact, seems to me to be utterly absurd. Lepidopterologists "formulate theories and marshall reasons to support them" and "consider objections and try to meet these" and "construct arguments against other views"; how foolish it would be to conclude from this that lepidopterologists are really interested in reasoning rather than butterflies. I really do have an interest in reasoning -- it's one of the things that fascinates me about the history of philosophy although I would never say that it is what I "really love" -- and I yet constantly have difficulty getting my colleagues to step back from whatever argument they are having at the moment and look at the general patterns or strategies of reasoning involved in a discussion. And it's pretty clear that everyone has strong preferences for the particular kinds of reasoning they try to use; almost everyone leaves lots of reasoning out (as Nozick wishes to leave out many 'proclamations and aphorisms'), focusing only on what they've been trained to work with; which is not what you'd expect if reasoning were really the interest here. It is certainly true that there are philosophers who love reasoning; just as no doubt it is true that there are carpenters who love hammers; just as no doubt it is true that there are chemists who love pouring liquids into glass containers. It is also not to any serious point.