Visible unities as a hint of the Trinity's one substance
Augustine is explaining how three things can share one name and one substance, using visible examples to prepare for the claim that Father, Son, and Holy Spirit are one God.
And it is no wonder that such things are said about the ineffable nature, since something similar happens even with the things we see with bodily eyes and judge by bodily sense. For when we are asked about a spring, we cannot say that it is the river; and when we are asked about the river, we cannot call it the spring. Again, the drinking water that comes from the spring or the river we can call neither river nor spring. Yet in this trinity we name water, and when we are asked about each one separately, we answer 'water' for each. For if I ask whether there is water in the spring, the answer is water; and if I ask whether there is water in the river, nothing else is answered; and in that drinking water there can be no other answer. Yet we do not say there are three waters, but one.
Of course, one must beware lest anyone think of the ineffable substance of that majesty as being like this visible and bodily spring or river or drinking water. For in these, the water that is now in the spring flows out into the river and does not remain in itself; and when it passes from the river or the spring into the drinking water, it does not stay where it was taken from. So it can happen that the same water now belongs to the name of spring, now to the name of river, now to the name of drinking water. But in that Trinity we have said it cannot happen that the Father himself is sometimes the Son, sometimes the Holy Spirit.
In a tree, the root is nothing but root, and the trunk is nothing other than trunk, and we can call the branches nothing but branches. For what is called root cannot also be called trunk and branches; and the wood that belongs to the root cannot by some passage be now in the root, now in the trunk, now in the branches, but only in the root. Yet the rule of the name remains: the root is wood, the trunk is wood, the branches are wood, and still they are not called three woods, but one.
Or if these have some dissimilarity, so that they could without absurdity be named three woods—since they can indeed be called three woods because of the difference in their solidity—then certainly everyone grants this: if three cups are filled from one spring, they can be called three cups, but they cannot be called three waters. They are altogether one water, even though when asked about each cup individually you would answer that there is water in any one of them, although no passage takes place here as we said happens from the spring to the river.
But these bodily examples have been given not because of any likeness to that divine nature, but because of the unity that exists even among visible things. They are given so that it may be understood that it is possible for some three things to obtain a singular name not only one by one, but even all together. Then no one should be amazed or think it absurd that we call God the Father, God the Son, God the Holy Spirit, and yet not three gods in this Trinity, but one God and one substance.
Latin
Nec mirum, quod haec de ineffabili natura dicuntur, cum in his etiam rebus, quas corporeis oculis cernimus et corporeo sensu diiudicamus, tale aliquid accidat. nam cum de fonte interrogati non possumus dicere, quod ipse sit fluuius, nec de fluuio interrogati possumus eum fontem uocare, rursus potionem, quae de fonte uel fluuio est, nec fluuium possumus appellare nec fontem, tamen in hac trinitate aquam nominamus et cum de singulis quaeritur, singillatim aquam respondemus. nam si quaero, utrum in fonte aqua sit, respondetur aqua; et si quaeram, utrum aqua sit in fluuio, nihil aliud respondetur, et in illa potione non poterit alia esse responsio; nec tamen eas tres aquas, sed unam dicimus. sane cauendum est, ne quisquam ineffabilem illius maiestatis substantiam sicut fontem istum uisibilem atque corporeum uel fluuium uel potionem cogitet. in his enim aqua illa, quae nunc in fonte est, exit in fluuium nec in se manet; et cum de fluuio uel de fonte in potionem transit, non ibi permanet, unde sumitur. itaque fieri potest, ut eadem aqua nunc ad fontis appellationem pertineat, nunc ad fluuii, nunc ad potionis, cum in illa trinitate dixerimus non posse fieri, ut pater ipse aliquando sit filius, aliquando spiritus sanctus. sicut in arbore radix non est nisi radix nec robur est aliud quam robur nec ramos nisi ramos possumus dicere; non enim quod dicitur radix, potest dici et robur et rami; nec lignum, quod pertinet ad radicem, potest aliquo transitu nunc in radice esse, nunc in robore, nunc in ramis, sed tantummodo in radice, cum illa regula nominis maneat, ut radix lignum sit et robur lignum et rami lignum, nec tamen tria ligna dicantur, sed unum. aut si haec habent aliquam dissimilitudinem, ut possint non absurde tria ligna nominari, ut etiam tria ligna dici possunt propter firmitatis diuersitatem, illud certe omnes concedunt, si ex uno fonte tria pocula inpleantur, posse dici tria pocula, tres autem aquas non posse dici, sed omnino unam aquam, quamquam de singulis poculis interrogatus in quolibet horum aquam esse respondeas, quamuis nullus hic transitus fiat, sicut de fonte in fluuium dicebamus. sed haec non propter illius diuinae naturae similitudinem, sed propter uisibilium etiam unitatem corporalia exempla data sunt, ut intellegeretur fieri posse, ut aliqua tria non tantum singillatim, sed etiam simul singulare nomen obtineant, ne quisquam miretur et absurdum putet, quod deum dicimus patrem, deum filium, deum spiritum sanctum nec tamen tres deos in ista trinitate, sed unum unamque substantiam.