  Nevertheless, Boltzmann remained skeptical about the validity of his hypothesis. For this reason, he attempted to explore different routes to his goal of characterizing thermal equilibrium in mechanics. Indeed, both the preceding (1871a) and his next paper (1871c) present alternative arguments, with the explicit recommendation that they avoid hypotheses. In fact, he did not return to this hypothesis until the 1880s (stimulated by Maxwell's 1879 review of the last section of Boltzmann's 1868 paper). At that time, perhaps feeling fortified by Maxwell's authority, he would express much more confidence in the ergodic hypothesis (see Section 5). 