Zinn's primary thesis in the quote is to reflect how some of the worst elements in history have happened because of obedience. For Zinn, the retelling of history as a sanctification and "white- washing" of awful truths have been done so through this "learned sense of moral proportion:"
One reason these atrocities are still with us is that we have learned to bury them in a mass of other facts, as radioactive wastes are buried in containers in the earth. We have learned to give them exactly the same proportion of attention that teachers and writers often give them in the most respectable of classrooms and textbooks. This learned sense of moral proportion, coming for the apparent objectivity of the scholar, is accepted more easily than when it comes from politicians at press conferences. It is therefore more deadly.
This "learned sense of moral proportion" is what Zinn believes helps to make awful historical truths happen again and again. Zinn believes that obedience to whatever the state and those in the position of power do is reflective of the manner in which history is taught. Zinn believes that if history is taught in a consensus mindset where there is no questioning of authority and no sense of examining preconceived notions and valences of power, atrocities and injustice will continue. The "learned sense of moral proportion" is a way to communicate the level of desensitizing that takes place towards atrocities and human brutality. This "learned sense" arises from the "most respectable of classrooms and textbooks." Zinn's primary idea behind the "learned sense of moral proportion" is to illuminate how historical consciousness is a battle that must be fought. Individuals must seek to raise questions, engage in dialogue, and embrace dissent as a natural part of their being. If not, the dangers of what the future will hold shall be evident in the "learned sense of moral proportion" that operates to keep citizens silent and embolden those in the position of power. It is from this point of view that guides Zinn and his scholarship, and keeps a light on a transformative vision of reality that seeks to make what is into what can be.