"close friendships define the contours of the participants' lives" Contours is a metaphor that comes from maps. Contours are the lines that show the shape of the land on the map. So when used as a metaphor like this, it means something like "if you were to make a map of this person's life, the features that would be most important would be their close friendships." "one of definition" Let's first restructure the sentence to put the important parts back together: "Some thinkers struggle to recognize this trend, and one of the reasons may be one of definition." So it is saying that one of the reasons that people struggle to recognise the trend may be the way that the trend is defined. Sometimes the way that something has been defined or described may be incomplete or not very clear, and that may prevent people from recognising something as a pattern or a trend. With a better definition or description, the truth may have been more obvious to them. "There are few numerical limits on the first 2 kinds---I can have a vast business network and hundreds of agreeable acquaintances." I assume that this follows on from the description of Aristotle's division of friendships. If so, it is saying that for the first two of Aristotle's three categories, it is possible to have very large numbers of such friends. It then re-words the categories to make the point. So "friendships for usefulness" could for example be the whole network of people I know in business, and I can have huge numbers of them. And "friendships for pleasure" are not necessarily deep and intimate friendships, they can be just "agreeable acquaintances", people I know and like. I can have lots and lots of them too. The paragraph will probably then continue to say that Aristotle's third category, "friendships of virtue", is a much rarer type of friendship, and you can never have as many of those.