Explain the grammar of "That not one fleecy lamb ..." in Keats' "Endymion"

Question

I have a question regarding the meaning of a stanza from Keats' Endymion:

Among the shepherds, ’twas believed ever,
That not one fleecy lamb which thus did sever
From the white flock, but pass’d unworried
By angry wolf, or pard with prying head,
Until it came to some unfooted plains
Where fed the herds of Pan: ay great his gains
Who thus one lamb did lose.

I am not sure which is the main clause is in the above sentence whose subject is 'not one fleecy lamb'. In other word, I don't understand the meaning to the above stanza; I am confused about the end of the stanza - from 'ay great his gain...'. Can someone please explain?

Answer 1

The main verb here is "pass'd", and what you're missing is the now rather archaic (and in this case divided by an intervening phrase) usage of the phrase "not one but". I can't find a good dictionary reference for this, but, in the English of Keats's time, "not one but did X" meant (and still means, but so rarely used now as to be almost obsolete) "everyone did X".

I'd guess, without proof, that this was probably originally a holdover from the French "ne ... que", which is similar, word by word, to "not ... but", and which functions as a sort of double negation to mean "only".

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You can find this usage in various other older pieces of literature, including 20th-century ones, several of which have previously been asked about on Stack Exchange:

One by one he subdued his father's trees
By riding them down over and over again
Until he took the stiffness out of them,
And not one but hung limp, not one was left
For him to conquer.

-- Robert Frost, "Birches" (1915), previously asked about on Literature SE

Peruse them well:
Not one of those but had a noble father.

-- William Shakespeare, All's Well That Ends Well (1623), previously mentioned on Literature SE

There were nearly twenty-five million inhabited planets in the Galaxy then, and not one but owed allegiance to the Empire whose seat was on Trantor.

-- Isaac Asimov, Foundation (1942), previously asked about on English Language & Usage SE

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So your Keats passage could be rephrased in more modern language as:

The shepherds always believed
That every fleecy lamb which thus departed
From the white flock would pass unharmed
By angry wolf or preying leopard,
Until it came to some untrodden plains
Where Pan's herds fed: everyone gained greatly
If he lost a lamb like this.

Answer 2

The difficult word here is “but”, which Keats uses in the following sense. (All definitions in this answer come from the Oxford English Dictionary.)

but, conj., 10.e.(b) With the pronominal subject or object of the subordinate clause unexpressed, so that but acts as a negative relative: that…not, who…not (e.g. Not a man but felt this terror,† i.e. there was not a man who did not feel this terror, they all felt this terror). Now archaic and rare.

_{† The dictionary’s example comes from Lamia by Keats, showing that the poet employed the word in this sense.}

So the main verb of the sentence is “passed” and if we temporarily remove the subordinate clause “which thus did sever from the white flock” then we get “not one lamb … but passed”, that is, the shepherds believed that all the lost lambs passed to the fields of Pan, unworried by wild animals. For “unworried” we need this sense of “worry”:

worry, v. 3.a. To seize by the throat with the teeth and tear or lacerate; to kill or injure by biting and shaking. Said e.g. of dogs or wolves attacking sheep, or of hounds when they seize their quarry.

The meaning of “ay” is uncertain—it could be “yes” (some editors put a comma afterwards to force this meaning) or it could be “always, ever”. Either way, the implication, I think, is that the shepherds believed that their lost lambs were offerings to Pan, the god of shepherds, who rewarded them with “great gains” in the size of the their flocks.