[Revised entry by Karen Neander and Peter Schulte on December 23, 2020. Changes to: Main text, Bibliography] Consider, for example, the thought that blossoms are forming. On a representational theory of thought, this involves a representation of blossoms forming. A theory of mental content aims to tell us, among other things, why this representation has this content, and so why it is a thought about blossoms forming, rather than about the sun shining, pigs flying, or nothing at all. In general, a theory of mental content tries to explain why mental states, events or processes (or, assuming a representational theory of them, the mental...