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Only by being forced to defend an idea against the doubts and contrasting views of others does one really discover the value of that idea.

What is the function of "does" in that sentence?

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  • See also "cleft sentence" and "do-support".
    – John Lawler
    Commented Apr 6, 2015 at 14:13
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    @EdwinAshworth But the answers on that page don't even explain why the inversion's necessary!!
    – Araucaria - Him
    Commented Apr 6, 2015 at 14:59
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    @Araucaria David Garner (below) gets as far as 'Without 'does', you'd have to say 'Only by ... views of others DISCOVERS ONE ...' which isn't done in modern English'. Which seems a repeat of Cool Elf's answer. But do you have an explanation of why 'it isn't done in modern English'? (Not the conditions for inversion you've now deleted.) I think that's going to be a long time coming.
    – Edwin Ashworth
    Commented Apr 6, 2015 at 15:17
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    @Araucaria I've upvoted your answer; it's a good summary of the principles involved in inversion, both this type and more generally. 'Explaining why the inversion's necessary' is nigh-on impossible; 'explaining the conditions which always trigger the inversion' is what you cover.
    – Edwin Ashworth
    Commented Apr 6, 2015 at 15:40
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    @Kris Araucaria's answer goes way beyond basics. How much of it OP would be happy with is another matter. It's perhaps another case of 'What you should be asking is ...' or at least 'What we wish you'd asked is ...'.
    – Edwin Ashworth
    Commented Apr 6, 2015 at 16:04

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It's to support subject/verb inversion (as in "neither do I" or "so do we"). So 'does' is standing in for the main verb, 'discover' because modern English avoids inverting except with auxiliary verbs such as do, can, will. Without 'does', you'd have to say 'Only by ... views of others DISCOVERS ONE ...' which isn't done in modern English.

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Only by being forced to defend an idea against the doubts and contrasting views of others does one really discover the value of that idea.

This sentence has certain properties which mean that there must be subject-auxiliary inversion in the main clause for the sentence to be grammatical. (This just means that the auxiliary verb and the subject change places.) These conditions are:

  1. It has an adjunct (read adverbial) .
  2. The adjunct has been fronted to the beginning of the sentence; it occurs before the main clause.
  3. This adjunct is being modified by the word only.
  4. Only has the meaning except A, not B here.

In the original sentence, the adjunct is a preposition phrase headed by a preposition is: by being forced to defend an idea against the doubts and contrasting views of others. This has been moved to the front of the clause and is being pre-modified by the adverb only.

Because, and only because, all four of these conditions are met, the subject and auxiliary verb in the main matrix clause must be inverted. Notice that this is not merely a case of negative adverb pre-posing. It is not quite the same as sentences such as:

  • Never have I seen such amazing paintings.

In the sentence above, the adverb never has been moved from the post auxiliary position to the beginning of the clause, it is not modifying an adjunct, it is the adjunct and it modifies the whole main clause. In the original poster's sentence, only modifies the preposition phrase adjunct "by being forced to defend an idea against the doubts and contrasting views of others". If we try to pre-modify a main clause with only, the result will not be grammatical.

  • I took some oranges.
  • I only took some oranges.
  • *Only did I take some oranges. (wrong)

Or, alternatively we will find that only has a different meaning serving as a discourse marker and we won't see any subject auxiliary inversion:

  • If we had some bacon, we could have bacon and eggs. Only we don't have any eggs!
  • *If we had some bacon, we could have bacon and eggs. Only don't we have any eggs! (wrong)

Here are some examples of bona fide subject auxiliary inversion with only:

  • Only if we've received the papers can we release the prisoner.
  • Only after the concert did I notice Pavarotti in the back row.
  • Only in extreme circumstances did they steal.
  • Only in the houses of parliament will you find this many cads.

If we don't have inversion here the sentences won't be grammatical:

  • *Only if we've received the papers we can release the prisoner. (wrong)
  • *Only after the concert I saw Pavarotti in the back row. (wrong)
  • *Only in extreme circumstances they stole. (wrong)
  • *Only in the houses of parliament you will find this many cads. (wrong)

In the Original Poster's sentence the main clause is in the present simple. If there was no inversion we would expect it to read:

  • One really discovers the value of that idea.

Because of the pre-posing of the only-preposition phrase we need subject auxiliary inversion. You will notice that in the main clause above there is no auxiliary verb, because it uses the present simple. When we need an auxiliary in such cases we use the dummy auxiliary DO. This gives us the necessary auxiliary verb to invert with the subject:

  • one does really discover the value of that idea ---> does one really discover the value of that idea.

In short then the function of does here is to allow subject auxiliary inversion. This inversion is necessary because of the fronting of a preposition phrase adjunct pre-modified by only.

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  • Overkill, IMHO.
    – Kris
    Commented Apr 7, 2015 at 5:28
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    I agree that this is different from negative adverb preposing, but I’m not sure I agree with the paragraph where you say that the difference is that “only modifies the subordinate phrase, but the inversion occurs in the main clause”. The subordinate phrase is in the main clause, surely, when the subordinate phrase is not a clause itself, no? Compare “Never did I understand it” to “Only then did I understand it”: syntactically, the adverbial phrases are both just preposed constituents in the main clause, not in a clause separate from it.
    – Janus Bahs Jacquet
    Commented Apr 7, 2015 at 9:06
  • Actually, come to think of it, I’m not so sure this isn’t just negative adverb preposing. Negating a prepositional phrase with not forces the same inversion (“not for nothing do they call him…”), so perhaps adverbial constituents modified by only just count as negatives, forcing inversion?
    – Janus Bahs Jacquet
    Commented Apr 7, 2015 at 9:12
  • @JanusBahsJacquet Well, I don't think so, mainly because you can't say Only have I taken some oranges, but you can say Never have I taken take any oranges. In other words, only has to be modifying some kind of pre-posed sentence adjunct.
    – Araucaria - Him
    Commented Apr 7, 2015 at 9:18
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    I think it's accurate now. It's a strange behaviour; 'Even [hours] after the concert, I saw Pavarotti in the back row' does not require/allow inversion (in today's English). You can have a go at 'Except Pavarotti, they'd all left' next (but this one will need authoritative references).
    – Edwin Ashworth
    Commented Apr 28, 2015 at 16:15

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