One ought, moreover, to recognise that it is one thing to look at a matter as it is, and another thing to look at it in the light of reason and thought. In the case of all created things, the distinction of the subsistences is observed in actual fact. For in actual fact Peter is seen to be separate from Paul. But the community and connection and unity are apprehended by reason and thought. For it is by the mind that we perceive that Peter and Paul are of the same nature and have one common nature. For both are living creatures, rational and mortal: and both are flesh, endowed with the spirit of reason and understanding. It is, then, by reason that this community of nature is observed. For here indeed the subsistences do not exist one within the other. But each privately and individually, that is to say, in itself, stands quite separate, having very many points that divide it from the other. For they are both separated in space and differ in time, and are divided in thought, and power, and shape, or form, and habit, and temperament and dignity, and pursuits, and all differentiating properties, but above all, in the fact that they do not dwell in one another but are separated. Hence it comes that we can speak of two, three, or many men.

And this may be perceived throughout the whole of creation, but in the case of the holy and superessential and incomprehensible Trinity, far removed from everything, it is quite the reverse. For there the community and unity are observed in fact, through the co-eternity of the subsistences, and through their having the same essence and energy and will and concord of mind, and then being identical in authority and power and goodness —” I do not say similar but identical —” and then movement by one impulse. For there is one essence, one goodness, one power, one will, one energy, one authority, one and the same, I repeat, not three resembling each other. But the three subsistences have one and the same movement. For each one of them is related as closely to the other as to itself: that is to say that the Father, the Son, and the Holy Spirit are one in all respects, save those of not being begotten, of birth and of procession. But it is by thought that the difference is perceived . For we recognise one God: but only in the attributes of Fatherhood, Sonship, and Procession, both in respect of cause and effect and perfection of subsistence, that is, manner of existence, do we perceive difference . For with reference to the uncircumscribed Deity we cannot speak of separation in space, as we can in our own case. For the subsistences dwell in one another, in no wise confused but cleaving together, according to the word of the Lord, I am in the father, and the father in Me: nor can one admit difference in will or judgment or energy or power or anything else whatsoever which may produce actual and absolute separation in our case. Wherefore we do not speak of three Gods, the Father, the Son, and the Holy Spirit, but rather of one God, the holy Trinity — Book I, Chapter 8, Concerning the Holy Trinity.