The following speculations do not pretend to state a complete doctrine of the Trinity, though they do state decisive parts of one. The first section is an account of what is, in my view, the function of a Trinity-doctrine, and will to some extent repeat work also published elsewhere. The second section is an interpretation of the trinitarian word ‘hypostasis’, that tries to display the conceptual revolution made by the traditional trinitarian assertion of ‘three hypostases of God’. The third considers the oneness of God, by analysing a famous argument of Gregory of Nyssa. And the fourth draws some further conclusions.