William Cronon is talking about people who oppose liberal education. These people believe liberal education is in place to brainwash people and create ideological zealots that spout brainwashed ideals. This is the only purpose of liberal education in the minds of these skeptics, mainly because they have been brainwashed by their own ideals and cannot see the other side of the argument.
The other side of the arguement is that liberal education is in place to help peoples minds fluorish. This is done by subjecting people to a wide array of opinions and points of view so the student can see all sides of the problem or arguement. By educating people this way they become true scholars with a genuine passion for learining and knowledge. Cronon uses evidence of how people who are educated in a liberal fashion have the ability to listen, read, talk, write, and solve problems more effectively than others.
I find this arguement to be very persuasive because without considering all points of view you become narrowminded, to truly learn you must open your mind to all possiblities.