Axiem wrote: ↑01 Mar 2018 20:33
I'm trying to think of an instance of that meaning outside of oaths, and I'm struggling to think of any. It feels more like "by" gained that meaning as a result of being translated from "pro"—so I'm wondering how it got picked instead of "before" or "for" or anything like that.
Axiem wrote: ↑01 Mar 2018 18:43But also, what does it mean to swear in front of Jove? Why would that matter?
Jove is the head of the Roman pantheon. If your assertion is endorsed by the king of the gods, that lends some credibility.
I'm aware of who Jove is; that I named him instead of anything else is immaterial to my query. What does it mean to swear in front of [literally anything]? Why would that matter?
Because Jove has the power to smite you with thunderbolts if you displease him.
There's a more general Indo-European (? at least?) cultural phenomenon here of the authoritative witness: the idea that things only have authority when they are witnessed by an authority figure. So, for example, weddings traditionally take place in front of a priest, and even secular weddings now take place in front of a government-appointed witness. In the past, marriages might be conducted in front of a local lord, or a monarch, for example, to aid weight to them. We even say metaphorically that someone is married "in the eyes of God", "in the eyes of the Church", or "in the eyes of the State". Similarly, to stress honesty, you can exclaim "as God is my witness!" and the like. In like fashion, in ye olden times, sometimes other important contracts would be signed (etc) in front of authority figures.
I think the underlying conventions here are that the witness is expected to at the very least tell people about the agreement, and in some cases it seems is expected to try to actually enforce it, and that when you break a promise it's a crime not just against the other party, but against the witness. When you ask someone to stand as a witness for a promise you make, you're leaning on their authority and making them collaborators in the agreement; when you then show that you were lying all along, you at best humiliate the witness (for not having realised you were lying) and at worst dishonour them (because they've been telling people, either actively or just through the fact of their 'endorsement' that you're going to do something, and you've made them unknowingly lie for you).
So calling on someone powerful, like God, as a witness to a promise shows seriousness: if you break the promise, you're showing your contempt for God, and risking divine retribution. Conversely, if you have God as your witness and someone still doubts you, then they're doubting God...
There may also be a reinforcement of this power in English by conflation with instrumental uses of 'by'. For isntance: "By God's love, you are healed!" is an instrumental (/or passive) (God heals you), but can also be reanalysed as an oath or exclamation ("I swear that you are healed").
Then, though, you have the trickier case of swearing by a loved but not necessarily powerful thing. Partly this may be simple bleed-over from swearing by a (presumably beloved, if you're loyal/pious) authority. Partly, I wonder if it's about shame: just as to ask Bob to witness for you when you're intending to lie is an insult to Bob because you are implicating him in your deception, there's probably a sense here that, say, calling your dead grandmother as your witness (or, metonymously, her grave) would disgrace her and insult her if you are in fact lying, because, again, you're implicating her in your crime. Plus, there's also of course the factor that swearing by someone or something you love and lying would shame YOU "in their eyes" - swearing by your children, for instance, and then lying, in this sense would mean them seeing you disgrace yourself.
But pretty clearly there's also a huge conflation going on here between this sense of swearing 'by' (in front of, as a witness), and the alternative of swearing 'on' something, in the sense of them being a hazard at forfeit. Sometimes 'on' has the meaning 'by' here: swearing on the Bible basically just means swearing by, in front of, etc, the Bible. But if you swear "on my life", or "on the lives of my children" or "on my hope of redemption" or whatever, then the idea underlyingly is that if you lie, those things (your life, their lives, etc) are forfeit. This sense of hazard, and more generally a sense of 'X for Y', crops up in many uses of 'on' (a fight that's "two on one", a wager that's "six thousand on red", interest that's "ten percent on the principal, year on year", etc).
There are cases where the boundaries between the two senses (witness and forfeit) blur - "on my honour" (at cost of my honour if I lie) and "by my honour" (my honour stands as my witness), for instance, and I think now basically both words can be used where the other sense originally applied. So "I swear by my children's lives!" probably originates through conflation with swearing ON your children's lives.
Now, of course, "by" has become a simple introduction to an oath, by Vectron's kindly claw, and has been fixed in that sense regardless of the original derivation.
[while we're at it, "for"/"fore" can be used in these cases (often spelled 'fore, but that's probably a hypercorrection like 'till):
Then sware Sir Thomas Howard,
'Fore God, I am no coward
But I cannot meet them here,
For our ships are out of gear,
And half my men are sick.
I must fly, but follow quick.
We are six ships of the line:
Can we fight with fifty-three?
Then spake Sir Richard Grenville,
"I know you are no coward -
You fly them for the moment
But to fight with them again
But I've ninety men and more
Who are lying sick ashore
And I should count myself the coward
If I left them, my Lord Howard,
To those Inquisition dogs
and the devildoms of Spain!
]