To read Donald MacKinnon’s theology with the presupposition that he was a systematic theologian must be a great disappointment. Reading MacKinnon leaves one with the impression that his work is rather unfinished, yet this is its quality. Embodied in his work is the belief that any system cannot in the end do justice to the realm of irreducible fact. There is a sustained rigour in his writing which is so deep as to give the reader the sense that MacKinnon is involved in an interrogation so penetrating as to be at times harrowing in its execution. George Steiner, quite rightly, speaks the word ‘sombre’ of MacKinnon’s work, but I think ‘interrogative’, with the accent on the restless even painful questioning of MacKinnon’s genius to be the better description. If MacKinnon can be said to have a method it is Baconian. The question rather than the thesis is the cutting edge of his theology. There is a Barthian insistence about his thought which wants to put all understanding, all subjectivity to the question in order to disclose a deeper level: the subjective must give way to the ontological. Implicit in his writings is the belief that to be a theologian is to be a realist: to be sensitive to the limits of understanding, to let God be God.