Abstract: A composite condition is one that consists of the conjunction of a purely internalist condition and a purely external condition; a condition that is not composite is prime. A general method of argument is provided for showing that many mental states, including knowing, are prime. A connection is made with the account of knowledge that Plato gives in the Meno. The primeness of mental states is shown to contribute to their value in the explanation of action by facilitating a kind of generality, which is made precise by use of the probabilistic notion of a correlation coefficient. The account is extended to the explanation of non-mental events.

Keywords: action, composite, condition, correlation coefficient, explanation, externalism, generality, internalism, Meno, Plato, prime,