Anselm compared the Trinity to the Nile. Water arises from a spring, travels as a river, and empties into the lake. As Dennis Ngien summarizes, “The spring is not the river nor is the lake; the lake is not the spring nor is the river. Yet the spring is the Nile; the river is the Nile; and the lake is the Nile.”

Not only is there a like/unlike duality here that captures some aspects of the Trinity, but for Anselm this helps to explicate and defend the filioque. Ngien again: “the spring does not exist from the river or from the lake; the river exists only from the spring, not from the lake; the lake exists from both the spring and the river.”

For Anselm, this image had another implication for the filioque:

Anselm claims that the Spirit proceeds from the Father and Son as from one principle, which means that the Spirit proceeds from what the Father and Son share, and not from them as distinct persons. For Anselm, “the lake does not exist from that by reason of which the spring and river differ from each other but exists from the water in virtue of which they are the same, so the Holy Spirit does not exist from that by reason of which the Father and Son differ from each other, but from the divine substance in which they are one. The origin is neither person taken by himself, but the divine essence which both persons possess.” If the Spirit proceeded from Father and Son at the level of person, then the differentiation of Father and Son would seem to be prior to the Spirit’s existence itself. But he is equally God, so the Spirit must be from the shared divine nature of the Father and Son.

But the analogy doesn’t hold the way Anselm wants. Contrary to his explanation, the lake doesn’t exist merely by virtue of the water that all three share. The lake exists because water has sprung up and moved in a particular way. The river exists only because the water first takes the form of a spring; and the lake exists because the water has taken the form of a spring and a river. This is a Western use of the analogy, but it’s one that takes the particularity of each person as being more ontologically fundamental than Anselm. Anselm speaks as if the divine nature itself existed, and could become the cause of the persons. But in the analogy there is no non-formed water, no “un-hypostatized” divine substance. The Son and Spirit are in their personal distinctiveness not because water exists but because the Father exists as spring.

(I am not endorsing the analogy as a whole. It captures one aspect of the order of life in the Trinity, but not the whole. If we can read off ontology from the economic event of Jesus’ baptism, then the Spirit is not the endpoint of a river, but is instead the river that flows into the reservoir of the incarnate Son, until the Spirit overflows to drench the whole body.)

The Eastern church had also used the Nile analogy of the Trinity, but did so in a way that denied the filioque. Ngien: “The Greeks reasons that although the lake comes from the river, it does not proceed from the river; rather it accumulates from the river.” Anselm rejects this argument because it assumes that the Son goes outside the Father in being begotten and because it assumes an “interval” between the procession from the Father and the procession through the Son: “the river flowing from the spring proceeds outside the spring and after an interval accumulates in a lake.”

Anselm responds with an appeal to the Creator-creature distinction. In created things, “principal source” means one thing; it means that the river flows outside the spring and then moves toward a lake. But in the uncreated life of God, Anselm argues, things are different. There is no before and after, and therefore the Son remains in the Father even while being begotten by Him and there is no interval. So, any model that assumes interval and an “outside” fails to apply. His own use of the analogy doesn’t fall prey to these flaws, since He doesn’t think that the Son moves outside the Father, or that there is any sort of space spring, river, and lake. It doesn’t take time for water to accumulate in the divine lake; it is always already full.

This point seems more secure than his earlier claims about the Spirit arising from the divine substance. But it is not entirely convincing. It does seem that there is some usefulness in “outside” and “interval” as applied to the divine life. Though the persons are perichoretically “inside” one another, there is also real differentiation, such that John can say that the Word is “toward” the Father in face-to-face communion. It is hard to see how the Triune God could create a world “outside” unless there was first some “outside” within Himself. And if there is an outside, there is also “interval” - not spatial nor temporal, but the divine interval of difference that is the uncreated ground of spatial and temporal intervals.