Armstrong's theory of universals treats relations as having no particular ontological difficulty, they can be treated in the same way non-relational properties are. How Armstrong's theory of universals deals with relations with varying adicities has been raised as an issue by Fraser MacBride. MacBride argues that there can be relations where the number of terms in the relation varies across instances. Armstrong's response is to affirm a theory he describes as the Principle of Instantial Invariance, wherein the adicity of properties are essential and invariant. According to Armstrong, complex relations which seem to challenge the principle are not ontologically real but are second-order properties that can be reduced to more basic properties that subscribe to the Principle of Instantial Invariance.