Aramis calculates d'Artagnan's loyalty like an engineer: “S’il a vu trop tard, il est Gascon et n’avouera jamais qu’il s’est trompé.” If the musketeer spots the substitution too late to act, his pride will do the rest — he'll defend the false king rather than admit he was fooled. The mechanism: don't persuade the proud man; arrange things so that admitting the truth costs him his self-image, and he will guard your secret for you.

— In our words · Source: The Man in the Iron Mask (Alexandre Dumas)