Historian: A democracy's citizens must know some history if the democracy is to meet its challenges.
Conclusion
Popular historical awareness is inevitably distorted.
Evidence
Most people learn history through popular narratives, and those narratives sustain interest by implying that a few famous heroes and notorious villains have shaped all of history.
Evaluate
The argument moves from "popular narratives imply X" to "popular awareness is distorted." That move only works if the implication itself is distorting — that is, if it is false (or distorting) to suggest a few heroes and villains have shaped all of history.
Goal
Find the assumption that the implication about heroes and villains is itself distorting. Negate it (the implication is accurate or non-distorting), and the conclusion collapses.