As the Pyrrhonians made clear, reasons that adequately justify beliefs can have only three possible structures: foundationalism, coherentism, and infinitism. Infinitism—the view that adequate reasons for our beliefs are infinite and non-repeating—has never been developed carefully, much less advocated. In this paper, I will argue that only infinitism can satisfy two intuitively plausible constraints on good reasoning: the avoidance of circular reasoning and the avoidance of arbitrariness. Further, I will argue that infinitism requires serious, but salutary, revisions in our evaluation (...) of the power of reasoning. Thus, reasoning can not provide a basis for assenting to a proposition—where to assent to a proposition, p, means to believe that we know that p. A non-dogmatic form of provisional justification will be sketched. Finally, the best objections to infinitism, including those posed by the Pyrrhonians, will be shown (at least provisionally!) to be inadequate. (shrink)