Writers the most learned, the most accurate in details, and the soundest in tendency, frequently fall into a habit which can neither be cured nor pardoned—the habit of making history into the proof of their theories.

Acton warns that even exemplary historians—erudite, meticulous, and morally serious—can succumb to a methodological vice: treating the past as raw material to validate a preconceived system. The target is not mere error in facts but a deeper distortion in purpose, where selection and emphasis are governed by theory rather than evidence. In Acton’s view, this habit is “neither…cured nor pardoned” because it undermines history’s critical function: to test ideas against the complexity of events and the moral agency of individuals. The remark reflects his broader commitment to intellectual honesty and to history as a discipline that should resist ideological capture.