The core of Oppenheim's attack on what he calls the natural law thesis is the contention that it rests upon an incorrect epistemology:
To subscribe to the natural law thesis is to adhere to the epistemological theory of value-cognitivism. Value-cognitivism claims that there exist intrinsic value-judgments which are cognitively true or false, regardless of the speaker's or listener's intrinsic value-commitments.
In contrast to this view is the epistemological theory of value non-cognitivism, which tells us that
Value-words do not designate objects, and it is misleading to use nouns such as “Justice” and “Goodness.” … A value-expression in an intrinsic value-judgment refers to a relation which holds between an evaluating subject and some object or event or state of affairs which he values intrinsically, whether positively or negatively.
I take the foregoing to mean that, to predicate just or good of a law or of a man does not tell us anything about the law or man, but rather describes an attitude toward the law or man. Justice, as a noun, is misleading, because justice is not a “thing” or a “this”; it is not a substance but an attribute; not a real noun, but an hypostatized adjective, a quality of evaluating subjects, never of the objects of which the subjects themselves always predicate it.