Identifying a source for this C.S. Lewis quote "This also is Thou: neither is this Thou"

Question

In C.S. Lewis's Letters to Malcolm: Chiefly on Prayer Letter 14 about different ways of conceptualizing God's presence among His creation, balancing between complete otherness and Pantheism. The quote is after this paragraph:

... All creatures, from the angel to the atom, are other than God; with an otherness to which there is no parallel: incommensurable. The very word "to be" cannot be applied to Him and to them in exactly the same sense. But also, no creature is other than He in the same way in which it is other than all the rest. He is in it as they can never be in one another. In each of them as the ground and root and continual supply of its reality. And also in good rational creatures as light; in bad ones as fire, as at first the smouldering unease, and later the flaming anguish, of an unwelcome and vainly resisted presence.

Therefore of each creature we can say, "This also is Thou: neither is this Thou."

Lewisiana.nl has an extensive and well researched Quotations and Allusions in C.S. Lewis, Letters to Malcolm which contains the following entry on that quote, suspecting that the quote's ultimate origin maybe from St. Augustine:

Charles Williams, He Came Down From Heaven (1938) ch. 2, p. 25; and The Descent of the Dove: A short history of the Holy Spirit in the Church (1939), p. 57. Lewis makes further use of these words in chapters 4 and 17. He was using it at least as early as 1942 in letter to Daphne Harwood (CL II, 512), where Walter Hooper mentions the chorus of Williams’s 1936 play Seed of Adam as one possible source.

While Lewis is undoubtedly referring primarily to Charles Williams, the real origin of this saying may be much older. “On the significance and authorship of this prayer, which Charles Williams may have found in St Augustine, see Victor de Waal, ‘The history of Doctrine’, Life of the Spirit, xviii (1964), 533” – thus Alastair Fowler in a note to C. S. Lewis’s posthumously published lectures, Spenser’s Images of Life (1967), p. 134.

Another good 2007 journal article on the same topic Contemplating C.S. Lewis's Epistemology by Norbert Feinendegen also mentioned St. Augustine as a possible source:

This last formula, "This also is Thou: neither is this Thou", Lewis borrowed from another of his friends, Charles Williams (who may in turn have read it in St. Augustine) (Lewis, "Williams and the Arthuriad" 151). Lewis accepted this concept as a necessary corrective to every purely affirmative theology (which stressed the unity of God and Man), as well as to every purely negative theology (which laid the stress on the otherness of God and Man) (Spenser's Images of Life 134).

Here's the quote from Lewis's "Williams and the Arthuriad":

Two spiritual maxims were constantly present to the mind of Charles Williams: ‘This also is Thou’ and ‘Neither is this Thou’. Holding the first we see that every created thing is, in its degree, an image of God, and the ordinate and faithful appreciation of that thing a clue which, truly followed, will lead back to Him. Holding the second we see that every created thing, the highest devotion to moral duty, the purest conjugal love, the saint and the seraph, is no more than an image, that every one of them, followed for its own sake and isolated from its source, becomes an idol whose service is damnation. The first maxim is the formula of the Romantic Way, the ‘affirmation of images’: the second is that of the Ascetic Way, the ‘rejection of images’. Every soul must in some sense follow both. The Ascetic must honour marriage and poetry and wine and the face of nature even while he rejects them; the Romantic must remember even in his Beatrician moment ‘Neither is this Thou’.

More explanation about Charles William's understanding of the two ways to God can be read from a Fr. Aidan Kimel's blog article The Iconic and the Apophatic: Charles Williams and the Two Ways.

However, it's possible that Charles Williams himself may have been mistaken, see the preface to his 1939 book The Descent Of The Dove:

... A motto which might have been set on the title-page but has been, less ostentatiously, put here instead, is a phrase which I once supposed to come from Augustine, but I am informed by experts that it is not so, and otherwise I am ignorant of its source. The phrase is: "This also is Thou; neither is this Thou." A a maxim for living it is invaluable, and it—or its reversal—summarizes the history of the Christian Church.

Question: Where in St. Augustine's work (or in another early church father's work) can we find this quote "This also is Thou: neither is this Thou" ?

Answer 1

If Williams had erred with his previous belief that the formula comes from St Augustine, its source may be a "Dogmatic Poem" of St Gregory of Nazianzus (ca. 329-390):

...

Thou art the purpose of every creature.

Thou art unique.

Thou art each one and art not any.

Thou art not a single creature nor art thou the sum of creatures.

...

(Gregory Nazianzen Dogmatic Poems [No. 29 Hymnus ad Deum], PG 37,507-8).

Source: The Roots of Christian Mysticism, 2nd Ed. by Olivier Clément (see Google preview of the page containing the poem)

Complete Latin version of the poem from Patrologiae Cursus Completus - Series Graeca - Volume 37 - Gregory of Nazianzus 3:

O tu qui es supra omnia ; quo enim alio te fas sit sermone celebrare?
Quomodo oratio te laudabit? tu enim nullis verbis efferri potes,
Quomodo mens te aspiciet? tu enim nulla mente percipi potes.
Solus es ineffabilis ; utpote qui creasti quæcunque voce enuntiantur.
5 Solus es qui nosci non possis; quippe creasti quidquid mente percipitur.
Omnia te prædicant, et quæ loquuntur, et quæ non loquuntur.
Omnia te mente prædita et non prædita honorant.
Commune enim desiderium, communia omnium consilia
Circa te ; te omnes deprecantur : tibi cuncta
10 Quæ compositionem tuam intelligunt, tacitum concinunt hymnum.
Tibi uni omnia permanent : ad te cuncta simul festinant.
Et omnium finis es, et unus, et omnia, et nihil horum,
Non unum es, non omnia; qui omnia habes nomina, qui te appellabo,
Qui solus appellari nequis? altiora nubibus cœlorum tegmina
15 Quæ mens cœlestis penetrabit? propitius sis,
Qui es supra omnia, quo enim alio te nomine fas est celebrare?

English translation by Olivier Clément in his book (I added line break adjustment to match the Latin):

O thou who art beyond all, How canst thou be called by another name?
What hymn can sing of thee? No name describes thee.
What mind can grasp thee? No intellect conceives thee.
Thou only art inexpressible; All that is spoken comes forth from thee.
5 Thou only art unknowable; All that is thought comes forth from thee.
All creatures praise thee, Those that speak and those that are dumb.
All creatures bow down before thee, Those that can think and those that have no power of thought.
The universal longing, the groaning of creation tends towards thee.
Everything that exists prays to thee
10 And to thee every creature that can read thy universe Sends up a hymn of silence.
In thee alone all things dwell.  With a single impulse all things find their goal in thee.
Thou art the purpose of every creature. Thou art unique.  Thou art each one and art not any.
Thou art not a single creature nor art thou the sum of creatures; All names are thine; how shall I address thee,
Who alone cannot be named? ...
15 ...
Have mercy, O thou, the Beyond All; How canst thou be called by any other name?

Answer 2

I cannot say where in St. Augustine the phrase may be found, but there's a reference in Spenser's Images of Life to an essay by Victor de Waal, "The History of Doctrine", Life of the Spirit, xviii (1964), 533. This may give the answer.

I do not have the essay (for I have no access to the JSTOR materials), but here's a link: https://www.jstor.org/stable/43706718?seq=1