The paper in question is based on research my collaborator X and I did together some months ago. Our respective contributions to the research were around 2/3 for X and 1/3 for me. In terms of the actual text of the paper, X wrote about 1/3 and I wrote about 2/3.
During the writing of the actual paper, X decided to leave academia. I can no longer contact X: emails to the old address go undelivered and there is no forwarding address for physical mail. Attempts to find X through web searches and contact X through mutual acquaintances have been unsuccessful. X has thus not seen a complete draft of the paper, only the sections that were actually written by X.
In these circumstances, is it ethical for me to submit a paper with X's name on it, without a complete version having been checked by X and without X's approval?
If I do submit it, should it contain a note of the fact that X was unable to check the completed paper? I am considering the hypothetical possibility that I could have inadvertently introduced an error while writing the complete paper (and such an error might survive through peer-review and into publication); responsibility for such an error should be mine alone.
There is no possibility of separating out my contribution into a separate paper. Either the work has to be published as a whole or not at all.
I am in a field where alphabetical listing of authors is standard, so there is no question of the order of authors.