I've heard some philosophers of mind use the term 'singular content'- but what does that mean?

The usual term would be something like "singular proposition", as opposed to a "general proposition". A singular proposition is one that is about some particular object. For example, the proposition that the Dalai Lama is German is a singular proposition. A general proposition would be something like: One and only one person is the spiritual leader of Tibetan Buddhism, and that person is German.

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