Egan is specifically addressing the in principle ability to understand something given time and inclination. He says, "I suspect that something broadly similar applies to minds and the class of things they can understand: other beings might think faster than us, or have easy access to a greater store of facts, but underlying both mental processes will be the same basic set of general-purpose tools." A short attention span will stop a person from putting in the necessary time to understand something. If a person had the ability to learn quantum field theory but couldn't afford to buy the textbooks, would you still say "a limit is a limit"?