grammar - What is the function of "do" in the following sentence?

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?

Answer

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.