The quest to foretell the future is omnipresent in human affairs. A potential solution to this epistemological conundrum has emerged through mass collaboration. Motored by the Internet, prediction markets allow a multitude of individuals to assume a stake in a security whose value is tied to a future event. The resulting prices offer a continuously updated probability estimate of the event actually taking place. This paper gives a survey of prediction markets, their history, mechanics, uses, and theoretical foundation. We also review the literature surrounding their efficacy. Though there are shortcomings with prediction markets, as well as practical constraints, they hold out the prospect of improving the quality of organizational decisions and increasing the level of participation in the deliberative process. We also note that lessons can be drawn from these markets to guide the epistemological practices of disciplines and inquiries in which empirical methods are difficult to apply.

A comparison is made between some epistemological issues arising in computer networks and standard features of social epistemology. A definition of knowledge for computational devices is provided and the topics of nonconceptual content and testimony are discussed.

Some philosophers believe that when epistemic peers disagree, each has an obligation to accord the other's assessment the same weight as her own. I first make the antecedent of this Equal-Weight View more precise, and then I motivate the View by describing cases in which it gives the intuitively correct verdict. Next I introduce some apparent counterexamples–cases of apparent peer disagreement in which, intuitively, one should not give equal weight to the other party's assessment. To defuse these apparent counterexamples, an advocate of the View might try to explain how they are not genuine cases of peer disagreement. I examine David Christensen's and Adam Elga's explanations and find them wanting. I then offer a novel explanation, which turns on a distinction between knowledge from reports and knowledge from direct acquaintance. Finally, I extend my explanation to provide a handy and satisfying response to the charge of self-defeat.

The question I consider is this:

The Question: Can two people–who are, and realize they are, intellectually virtuous to about the same degree–both be rational in continuing knowingly to disagree after full disclosure (by each to the other of all the relevant evidence they can think of) while at the same time thinking that the other may well be rational too?

I distinguish two kinds of rationality–internal and external–and argue in section 1 that, whichever kind we have in mind, the answer to The Question is ‘yes’ (though that positive answer is less wholehearted in the case of external rationality). Then, in section 2, I briefly make some more general remarks about when discovering a disagreement provides a defeater and when it doesn't. In the final section, I consider an important objection to the answer given in section 1 to The Question.