The central claim of this thesis is that concepts, the components from which cognitively significant truth evaluable content (thought) is composed, are unstructured entities an account of whose individuation makes no essential reference to other concepts in the possession of the thinking subject or to any particular means by which the reference of the concept is identified by the thinking subject. This position is called Conceptual Atomism and contrasts with Inferential Role Semantics, according to which concepts are individuated by their (...) inferential roles or their conditions of warranted application. The structure of the argument is as follows. Firstly, a principle called the Transparency Principle is developed. This places constraints on the individuation of concepts across differing contexts. The Transparency Principle is then used to show that Inferential Role Semantics is false because it cannot provide a satisfactory account of cognitive dynamics; that is, of the conditions under which a concept is retained through changes in the epistemic state of the subject over a period of time. A version of Conceptual Atomism is then defended and it is shown that this theory yields the correct individuation of concepts. According to this theory the concepts of an individual subject are individuated in terms of referential episodes, episodes of ongoing reference to an object or property during which it is diachronically transparent to the subject that the same thing is being referred to. The more general notion of a referential practice is then used to account for the sharing of concepts by more than one person. Finally, a novel account of the thoughts expressed using indexical terms is defended in order to show that indexicals present no counterexample to Conceptual Atomism. This account of indexical thoughts is of some consequence in its own right. (shrink)