The subject is not mendacity but “getting—or putting--horizontal.” The distinction is confusing in English, because the past tense of one verb is the present tense of the other. What a pickle!
“I think I’ll lie down for a while.” [Correct]
“Just lay the book over there on the table.” [Correct]
Two points need to be made about the simple sentences above. First, it isn’t a matter of humans versus the whole rest of the world (books and hens and such) but a matter of intransitive versus transitive verbs. (I heard those groans!) Unless there’s an object, you want the intransitive verb, in this case, to lie. That takes us to the second point, which is that dogs, when relaxing in front of the fire, are lying there, not laying, and the correct wording of the training command is “Lie down,” not “Lay down.” When, on the other hand, there’s an object following the verb, an egg or a book or whatever, you want the transitive verb, the one that can carry or transport its object.
It wouldn’t be so bad if that were the end of the story—but then along comes the pesky past tense.
“I lay down to rest and fell asleep for an hour.” [Correct}
“He laid the book carefully on the table, as if it were an egg.” [Correct: The addition of the egg was just to see if you were paying attention.]
Suddenly the spelling and sound of lay is correct for a human but only because it is the past tense of to lie, not to lay, the past tense of the latter being laid! We could take lie and lay into deeper waters, but why bother? Something tells me the distinction may vanish in my own lifetime.
Often, quite honestly, I almost wish I didn’t know the rules for using these verbs correctly, because the knowledge makes it so annoying to hear the incorrect usage. Embarrass my fellow human beings with public correction, I will not.
Okay, what about “Now I lay me down to sleep,” some of you are asking. Could it be that the confusion in English arises from this very source, the childhood prayer? It’s a tricky question, but note that ‘me’ in there. The speaker of this sentence takes himself or herself not only as subject (“I”) but also as object (“me”). Strictly speaking, it would be more proper to use a reflexive pronoun, i.e., “I lay myself down to sleep,” but of course the rhythm would then be entirely lost. Poetic license! Whether for rhythm or for rhyme, verse and song often depart from strict grammar, and there it’s just fine.