As curiousdannii suggests try replaying "save" with "except".
Here is the original passage from Chapter 7, "On Work":
And I say that life is indeed darkness save when there is urge.
And all urge is blind save when there is knowledge.
And all knowledge is vain save when there is work.
And all work is empty save when there is love.
To help understand it better, I replaced "save when" with "unless":
And I say that life is indeed darkness unless there is urge.
And all urge is blind unless there is knowledge.
And all knowledge is vain unless there is work.
And all work is empty unless there is love.
Here is a paraphrase of the above without line breaks:
Without urges life is darkness. Without knowledge urges are blind. Without work knowledge is vain. And without love work is empty.
Gibran, K. The Prophet. 1923. Retrieved on June 29, 2019 from Internet Archive at https://archive.org/details/KahlilGibranTheProphet_201903/page/n13