The subject of this paper abounds in historical problems of an extremely intricate nature—some of which arise through the theologically close association of the doctrine of the Holy Spirit with the doctrine of the Holy Trinity; but others of which are rooted in a number of more particular controversies in which the Greek Fathers were involved concerning the Holy Spirit. Among the latter are to be found such questions as the precise identification of the persons against whom many of the Greek writings were directed, for example, who the ‘Tropici’ were, with whom Athanasius deals in the Letters to Serapion, whether ‘Pneumatomachi’ was a generic term used to describe a variety of different heretics, or a proper name referring to an identifiable group existing in one particular place, whether Macedonius was a Macedonian, and so on. Included in the intricate historical problems raised in our period, there is that of accounting for the revival towards the middle of the fourth century of an interest in the doctrine of the Holy Spirit— whether it was due to the developing influence of asceticism, or simply a reaction to a latter-day Arianism working itself out belatedly in heresy concerning the Holy Spirit, or more subtly, whether it was due to a necessity felt by the Church to give substance to the Person and Work of the Holy Spirit, now that the doctrine of the Trinity had replaced the doctrine of the Logos as her central and dominant doctrine.