 Some scholars believe that in Descartes’ brief discussion of primary ideas there is suggested a principle of representation (Wilson 1978, Clatterbaugh 1980, Chappell 1986, Smith 2005a, 2010a). The principle is notoriously difficult to formulate, and there is no consensus among scholars as to how it is best understood. However, a large number of scholars agree on one component of the principle, which can be expressed as a necessary (though not a sufficient) condition for representation. This principle of representation (PR) can be expressed as follows: