Chapter V.—Concerning the number of the Natures.
In the case, therefore, of the Godhead 1993 we confess that there is but one nature, but hold that there are three subsistences actually existing, and hold that all things that are of nature and essence are simple, and recognise the difference of the subsistences only in the three properties of independence of cause and Fatherhood, of dependence on cause and Sonship, of dependence on cause and procession 1994 . And we know further that these are indivisible and inseparable from each other and united into one, and interpenetrating one another without confusion. Yea, I repeat, united without confusion, for they are three although united, and they are distinct, although inseparable. For although each has an independent existence, that is to say, is a perfect subsistence and has an individuality of its own, that is, has a special mode of existence, yet they are one in essence and in the natural properties, and in being inseparable and indivisible from the Fathers subsistence, and they both are and are said to be one God. In the very same way, then, in the case of the divine and ineffable dispensation 1995 , exceeding all thought and comprehension, I mean the Incarnation of the One God the Word of the Holy Trinity, and our Lord Jesus Christ, we confess that there are two natures, one divine and one human, joined together with one another and united in subsistence 1996 , so that one compound subsistence is formed out of the two natures: but we hold that the two natures are still preserved, even after the union, in the one compound subsistence, that is, in the one Christ, and that these exist in reality and have their natural properties; for they are united without confusion, and are distinguished and enumerated without being separable. And just as the three subsistences of the Holy Trinity are united without confusion, and are distinguished and enumerated without being separable 1997 , the enumeration not entailing division or separation or alienation or cleavage among them (for we recognise one God the Father, the Son and the Holy Spirit), so in the same way the natures of Christ also, although they are united, yet are united without confusion; and although they interpenetrate one another, yet they do not permit of change or transmutation of one into the other 1998 . For each keeps its own natural individuality strictly unchanged. And thus it is that they can be enumerated without the enumeration introducing division. For Christ, indeed, is one, perfect both in divinity and in humanity. For it is not the nature of number to cause separation or unity, but its nature is to indicate the quantity of what is enumerated, whether these are united or separated: for we have unity, for instance, when fifty stones compose a wall, but we have separation when the fifty stones lie on the ground; and again, we have unity when we speak of coal having two natures, namely, fire and wood, but we have separation in that the nature of fire is one thing, and the nature of wood another thing; p. 50b for these things are united and separated not by number, but in another way. So, then, just as even though the three subsistences of the Godhead are united with each other, we cannot speak of them as one subsistence because we should confuse and do away with the difference between the subsistences, so also we cannot speak of the two natures of Christ as one nature, united though they are in subsistence, because we should then confuse and do away with and reduce to nothing the difference between the two natures.
Leont., Resp. ad argum. Sever.49b:1994
For καὶ τῇ αἰτιατῇ καὶ ὑ& 187·κῇ, καὶ τῇ αἰτιατῇ καὶ ἐκπορευτῇ we get καὶ τῇ αἰτιατικῇ, καὶ ὑ& 187·κῃ, καὶ πορευτῇ in Cod. Colb. 1, Cod. Reg. 3, and so Faber also.49b:1995
Leont., Resp. ad argum. Sever.49b:1997
See Leont., Act. 7. De Sect., with reference to one of the arguments of the Nestorians; also Greg. Naz., Orat. 36; Max., Ep. 1 ad Joan. Cubic.49b:1998
Infr. ch. vii.: Basil, Epist. 40 and Bk. De Spir. Sanct. ch. 17.