We think that certain of our mental states represent the world around us, and represent it in determinate ways. My perception that there is salt in the pot before me, for example, represents my immediate environment as containing a certain object, a pot, with a certain kind of substance, salt, in it. My belief that salt dissolves in water represents something in the world around me, namely salt, as having a certain observational property, that of dissolving. But what exactly is the relation between such states and the world beyond the surfaces of our skins? Specifically, what exactly is the relation between the contents of those states, and the world beyond our bodies?

I believe that the correct view of the relation between certain mental contents, the contents of at least some of our intentional states, and the world beyond our bodies is an externalist one. Crudely, externalism is the view that certain of our intentional states, states such as beliefs and desires, have contents that are world-involving. Less crudely, it is the view that certain intentional states of persons, states such as beliefs and desires, have contentful natures that are individuation-dependent on factors beyond their bodies. My belief that salt dissolves in water, has a content, that salt dissolves in water, that is individuation-dependent on a certain substance in the world beyond my body, namely, salt.

The roots of externalism lie in the work of Hilary Putnam, who was concerned to show something, not specifically about the nature of mental states, but about the nature of meaning.