We all ‘know’ that public opinion came to prominence in the political vocabulary of the late eighteenth century. It may be that this dates its rise a bit late, but it is not relevant to argue the matter here. My concern is rather that we be equally aware of the purposes for which people made use of the concept. Here I wish to consider various possible contexts for speaking or writing of public opinion, or ‘opinion’, as it was usually called prior to the mid-eighteenth century. It may be possible to define, more fully than heretofore, the work that the expression did in eighteenth-century thought. As contemporary students of public opinion have been learning, an answer to this question may not even be wholly irrelevant to the task of specifying the nature of public opinion in our own time.