A man with only one book, will arguably have mastered the content. Why isn’t that enough, to follow his leadership?
If I were to choose the oldest manual on war, “The art of war,” by Sun Tzu (“Sunzi”) I might miss significant developments in the theory of war, brought into focus by generals fighting Sun Tzu advocates. That alone should be enough to concern ourselves with both old material and new.
However, if I were to learn a principle that Sun Tzu taught, but make incorrect, or inadequate application of it, I might learn the same thing differently, or better, from Xenophon’s “The Education of Cyrus.”
In keeping with the observation that Generals benefit from extra curricular reading, such as psychology texts, the man with only one book may fight a costly war because of failure to incorporate new blood, from opposing but well articulated views.
It is proper to learn, even from an enemy – Ovid