All of us get opinions from other people. And not just a few. We acquire opinions from others extensively and do so from early childhood through virtually every day of the rest our lives. Sometimes we rely on others for relatively inconsequential information. Is it raining outside? Did the Yankees win today? But we also depend on others for important or even life preserving information. Where is the nearest hospital? Do people drive on the left or the right here? We acquire opinions from family and close acquaintances but also from strangers. We get directions from and heed the warnings of individuals we've never met, and likewise read books and articles and listen to television and radio reports authored by individuals we don't know personally. Moreover, we undertake inquiries in groups in which the group relies on the conclusions of the individuals making up the group. In some of these collective efforts everyone knows one another, for example, a set of neighbors taking a census of birds in the neighborhood. But others, such as the effort to understand gravity, are not so nearly self-contained. Indeed, many of the most impressive human intellectual accomplishments are the collective products of individuals far removed from another in location (and sometimes even over time) who rely on each other's conclusions without feeling the need to re-confirm them.