Donald MacKinnon's writings from about the mid–1960s onwards, at least, return regularly not simply to the fundamental theme of ‘realism versus idealism’, but to the treatment of this issue by G. E. Moore in a classical essay on ‘External and Internal Relations’. This, along with certain other writings by Moore and Russell in their assault on idealism, sets the terms of the problem for MacKinnon; and it is important to remember this when, as is sometimes the case, the words ‘realism’ and ‘idealism’ seem to become impossibly loose in their scope. Behind all the discussion of the question of fundamental ontology in MacKinnon's maturest work stands a set of rigorous arguments in logical theory, in the light of which this work requires to be understood; and without this perspective the heart of MacKinnon's theological achievement remains opaque.

Moore's target is primarily Bradley's contention that every relation in which a specific term is involved enters into the being of that term, so as to be intrinsic to it. Moore's first clarification (pp. 281–2) is to note that, strictly speaking, it is ‘relational properties’ that are in question, not relations – i.e., Bradley's claim is one about relations to particular and distinct terms that are truly predicated of another single term (‘A is the father of B’, not ‘A is a father’ only).