But since it doesn't mean that, we need some further implied meaning.
I'm a little bit confused what you and a few other people are trying to express. You said that you were certain that a hypothetical was wrong, with no explanation then or since.
Your inference that "you" within the scope of "while this is in play" would refer to the person who played a card rather than the person whose play area the card is in, in the hypothetical scenario that those could be different, is not well-supported in the text of this thread (even though Donald has said that, in a case where they were likely to be different, he would probably use more specific wording); and your conclusion that this would continue to apply even if it were possible for a card to be in play without anyone having played it is a further inference beyond that and even less supported. I don't see where your inference that Highway would have no effect in this scenario comes from at all, since its text doesn't even contain "you".
Well, I went with what Donald said. It could also be based on card ownership, but he didn't even mention card ownership.
I don't think what Donald said supports your point.
He said: "Lighthouse's text is addressing a specific player.... It is addressing the player who played it though."
I assume this is what you're thinking of when you say that Donald said the meaning of "you" in the scope of "while this is in play" is 'the person who played the card'. However, that's not actually what Donald said here—he said that the person being addressed is the person who played the card, but not that they're being addressed
by virtue of having played the card (instead of by virtue of having the card in their play area). The person who played the card
is the person with the card in their play area, so the fact that that person is being addressed—they are the semantic
extension of "you" on the card—does not disambiguate what the semantic
intension is.
Donald also said: "To be technically correct [cards like Lighthouse] would need to say e.g. 'while you have this in play.'" This
is a statement about the intension of how cards like Lighthouse are meant to be interpreted, and is incompatible with your hypothesis that a Lighthouse somehow placed in play without anyone playing it would not protect its owner. If you want you can consider this a miswording like the text of Nomad Camp; I don't agree with you on that, but I don't think the ruling is ambiguous in any case.
It could not be solely based on having the card in your play area (only as a result of card ownership, as stated by LastFootnote), because "in play" specifically means "in any play area". In other words, "while this is in play" applies to all players, but the "you" in the text that follows applies to a specific player. According to Donald, that player is the player who played the card.
The player who played the card
is the player whose play area it's in.
If you say that in "while this is in play", "in play" actually means "your play area", the Grand Castle ruling is very inconsistent.
Grand Castle doesn't say "while this is in play".
But I continue to insist that what's at issue here is
not the meaning of "in play" but the meaning of "you", because I think you're equivocating on whether "you" is to be treated as a bound or free variable (roughly speaking). The difference between "while this is in play" and "while this is in your play area"
as a statement of when a card ability is active is not in the condition that they establish, but in how the pronoun "you" is bound. The latter explicitly defines "you" (i.e., identifies the person being addressed) as the person with Lighthouse in their play area; and then "you" showing up later in the text would be
bound to that definition. The real text of Lighthouse, as you note, doesn't make that explicit definition of the variable, and so then when "you" shows up under the scope of "while this is in play" it's as an unbound pronoun and we need to figure out who it refers to; and the convention or implication is that it refers to the person whose play area it's in.
Contrast this with Grand Castle. Grand Castle says "when you gain this"—thus defining "you" as referring to the person who gains the card, and any future occurrences of "you" in the card text are bound to
that referent of "you". In
this context, "in play" and "in your play area" have very different meanings, because which "you" is
already a bound variable—i.e., because the referent of "you" has already been defined.
But on Lighthouse, "when this is in play" and "when this is in your play area" have the same functional meaning, because the referent of "you" is not yet bound to a specific player.