 There might be at least two kinds of diachronic obligation here: one governing how we form and hold beliefs over time, and the other governing how we relinquish or revise beliefs over time. If someone violates such a diachronic obligation by “purposely avoiding the reading of books and the company of men who call in question” his presuppositions, Clifford warns, then “the life of that man is one long sin against mankind” (1877, 77). 