In academic philosophy and in ordinary life, it is often difficult to tell whether two people are really communicating, agreeing, or disagreeing about a single subject--about the metaphysical status of concepts, say, or about the color of a certain garment--or whether the two people are instead merely `talking past' one another, respectively making claims about two different subjects. In attempting to make sense of these cases, it will often be helpful to ask the question of whether the two people are sharing a thought or are instead thinking different thoughts from one another. In many cases of interest, the distinction between true and merely apparent communication, agreement, or disagreement will come down to the distinction between sharing and not sharing a thought.But although this analysis is fairly commonsensical, it presupposes that sharing thoughts is common. If sharing thoughts were somehow to turn out to be extremely rare, it would become difficult to make sense of the distinction between true and merely apparent communication et cetera, since if they involve sharing thoughts then these phenomena would themselves be extremely rare. The meaningfulness of this distinction and of many other distinctions we make in philosophy and in ordinary life requires that we often share thoughts with one another.If concepts are the building blocks of thought, then sharing thoughts requires sharing concepts. So any theory of concepts that is intended to respect our ordinary way of thinking about ourselves should include an account of the publicity of concepts--an account of how it is that so many concepts are shared among so many different people. This enables the theory to give a reasonable analysis of everyday phenomena like communication, agreement, and disagreement.However, there is a large class of theories of concepts that includes many popular contemporary theories and that is incompatible with the publicity of concepts. This is the class of `individualistic' theories of concepts, so-called because they treat concept possession as something that only individuals do. These theories are incompatible with the publicity of concepts because they take many subtle features that distinguish individuals from one2 another to be relevant to questions about which concepts those individuals possess, and so they inevitably answer "no" to any questions about whether a certain individual shares a certain high-level concept with another individual.On individualistic theories, including the theories offered by Jerry Fodor and Christopher Peacocke, as well as others, the best picture that can emerge is one on which many individuals possess many concepts that are merely very similar to concepts possessed by others. But mere similarity cannot do the theoretical work required of concept sharing, so these individualistic theories are deeply incompatible with our ordinary way of thinking about ourselves.The solution is to adopt a `social' theory of concepts, which in contrast to an individualistic theory treats concepts as importantly socially articulated entities--as entities that have some social features at a more basic level than they have some other important features, such as their semantic values. Although social theories do face certain difficulties, it is possible to overcome these difficulties, in part by tying concepts very closely to public-language words and so accounting for the publicity of concepts together with the publicity of public languages like English. On balance, social theories should thus be preferred to individualistic theories if it is important to respect our ordinary picture of ourselves, because they allow accounting for the publicity of concepts but do not come with any serious, unavoidable theoretical costs.