You can persuade a man to believe almost anything provided he is clever enough, but it is much more difficult to persuade someone less clever.

The remark plays on a paradox about intelligence and credulity. Stoppard suggests that a “clever” person can be easier to talk into almost any position because they can supply their own sophisticated justifications—rationalizations, clever arguments, and interpretive frameworks that make a belief feel coherent. By contrast, someone “less clever” may be harder to persuade not because they reason better, but because they lack the rhetorical or conceptual machinery to dress up a new belief as plausible; they may rely on blunt common sense, habit, or simple skepticism. The line also satirizes intellectual vanity: the more agile the mind, the more capable it is of self-deception in the name of being reasonable.