Materialistic historiography . . . is based on a constructive prin-
ciple. Thinking involves not only the ﬂow of thoughts, but their
arrest as well. Where thinking suddenly stops in a conﬁguration
pregnant with tensions, it gives that conﬁguration a shock . . . In
this structure [the historical materialist] recognizes . . . a revolu-
tionary chance in the ﬁght for the oppressed past.