It is commonly assumed that mental states can be characterized by a certain psychological attitude and a certain content. The content of a mental state is a mental content. A precedent of that analysis can be found in Russell. Believing, desiring, remembering, feeling, perceiving, etc., are examples of psychological attitudes. What is believed, what is desired, what is remembered, what is felt, what is perceived, etc., are the mental content that in each case is associated with those attitudes.
Very often, it is also assumed that there are two big classes of mental contents: conceptual and non-conceptual ones. Conceptual content is the semantic content that we can find in words, expressions and sentences of a language. The content that beliefs, desires, rememberings, etc., typically have is the same as the content of certain sentences. Mental states with conceptual content are also called “propositional attitudes”, their content being a particular proposition that may be expressed by a certain sentence.
Non-conceptual content is a experiential, qualitative or phenomenological content. It is the alleged content that feelings, perceptions and sensations typically have. Whereas conceptual content is semantically evaluable in a quite direct way, non-conceptual content is not so. However, non-conceptual content can be evaluated as more or less correct or incorrect, or as more or less adequate or inadequate, etc. Mental states with non-conceptual content are usually called “qualitative states”, “experiential states” or “phenomenal states”. Their content eventually is a qualitative, experiential, or phenomenal character not identifiable with any proposition.
A very important thesis with respect to the distinction between conceptual and non-conceptual content is that perhaps there are mental states with both conceptual content and non-conceptual content. Another not least important thesis is that perhaps every mental state with conceptual content has also some kind of non-conceptual content.
The contrast between internism and externism has given place to one of the more dramatic discussions about mental content in recent years. Internism claims that mental contents –and mental states– only depend on factors internal to the mind of the subjects. Externism claims that mental contents –and mental states– essentially depend on factors external to their minds. Those external factors may include linguistic norms of the community, how experts would use certain terms, and the relationships with the external world. Descartes and Frege are two paradigmatic classical examples of internism. Externism was introduced by authors like Putnam, Burge and Kripke.
The bibliographic resources offered by David Chalmers in his website are extremely useful: <http://consc.net/chalmers/>