He guided the passions of others, because he was master of his own.

The sentence links effective influence over others to self-command. “Passions” suggests not only emotions but also impulses—anger, fear, ambition—that can sweep groups into rash action. Marsh’s point is that the person who can steady a crowd, lead a cause, or govern wisely does so not through manipulation alone, but through inner discipline: having mastered his own appetites and reactions, he can read and regulate those of others. The line also implies an ethical hierarchy—self-mastery as a prerequisite for legitimate authority—echoing a long moral tradition in which leadership begins with governing oneself.