In nautical (and aeronautical) contexts, a vessel may declare a number of "souls onboard". I've googled around for origin of that phrase, but haven't found a definitive explanation. Most discussions of the phrase I've seen on aeronautical forums and obviously concern the usage in aviation, with the discussion often revolving around on rescuers tallying the dead after an accident. But the phrase is obviously older than that and used in situations besides emergencies.
Of the explanations I've read, many claim that "souls" is used as it includes passengers and crew, whereas a number on a passenger manifest would only include the former. But even so "people", "humans", or simply "there are 150 onboard" would seem as effective.
While the inclusion of passengers and crew makes sense, I can't help but wonder what the requirements are for being counted as a "soul" and whether they've changed. Whether it's always meant "living human being" or if it was used to expressly exclude certain individuals rather than include them. For instance, would it include the (presumably non-babtized) "cargo" in the days of slave trade? A morbid thought, I admit.
It may just be tradition based in "souls" sounding more dignified than "humans", and the religion-laden language of its day. Yet I can't help but wonder.
Clarification: The answers here so far concern aviation usage, yet I simply mentioned aviation because that's the context in which I've seen the phrase discussed. But my question is about its original etymology; not its current usage. The phrase comes from a nautical context, but why did sailors use the word "souls"?
I understand the meaning of the phrase, and I understand that it makes sense as an inclusive term to mean "everyone onboard". I also understand that by now it's become traditional. But again the question is about its origin. Since a word like "persons" would be equally unambiguous (and as David Schwartz points out below, "persons" is indeed a valid alternative in modern aviation), why use "souls" instead? Was it for instance originally meant as an exclusive term?
Some of the explanations I've seen elsewhere:
- "Souls" excludes human remains being transported (in an aviation context this would help crash responders sort out crash victims from those already dead, but again the phrase predates aircraft and aircraft crashes)
- It refers back to "S.O.S" for "save our souls" (which must be wrong since that's not what S.O.S means. The code was chosen for its morse pattern and not as an acronym, and anyway the phrase is older than morse code too)
- "Souls" excludes lawyers and managers… (no comment)
By the way, in aviation it seems the phrase is sometimes abbreviated as "S.O.B", much to the delight of some pilots, especially when pluralized: "We've got 40 SOBs here. And two pilots."