nExpressions have both:
na reference: what it means on a given occasion (retinal image, subjective)
na sense: ways in which the reference is presented (lens image, objective)
nReference: actual real-world objects, individuals, classes (extension)
nU.S. President: {Washington, Clinton, Bush, …}
nSense: inherent meaning, concepts, thoughts (intension)
nU.S. President: elected leader of the U.S.