A donkey sentence is a kind of sentence that occurs in natural language, where an anaphoric pronoun (like hó) refers to a quantified variable (like sa ảqshe) even though it is outside of that variable's scope.
It is named after the following prototypical example of such a sentence:
Kỉaı tu hảqpaoche bö hóa sa ảqshe cy, áqshe da.
“Every farmer who owns a donkey, cares for it.”
The English sentence is clearly valid English. But the straightforward translation into logic of such a sentence is not well-formed:
⚠️ ∀f : Farmer(f) ∧ ∃d [Donkey(d) ∧ Owns(f, d)] → Cares(f, d).
The underlined instance of variable d is illegal, because it is outside of the scope of the quantifier ∃d.
Is the above Toaq sentence valid? What does it mean?
Hoemaı considers the case of anaphoric reference to a binding from a previous sentence not so different, and calls such pronouns donkey as well. There is then a "crude handling of donkey pronouns" demonstrated in On quantifiers and variables:
Kảqgaı jí sa gủoso da. Nủo gúoso da.
[∃G : guoso(G)] kaqgaı(J,G). [∀R : raı(R) ∧ guoso(R) ∧ kaqgaı(J,R)] nuo(R).
“I see some cows. They (the cows) are asleep.”
“I see some cows. All the cows that I see are asleep.”
Note the "surprising ∀ quantifier" in the logical translation of the second sentence. For as long as gúoso is logically out of scope, but not explicitly rebound, it refers to "all the cows that I see". “The point of it is to catch the same things as appeared in the previous sentence … instead of just any.”
Applying this same "crude handling" to quantifiers in subclauses, the meaning of the Toaq sentence above is: "All farmers who own some donkeys, care for all the donkeys that they own." This is often sufficient.
Hoemāı remarked that "donkey anaphora can't really be allowed in Toaq, unless you use a separate set of pronouns or a donkey conversion mechanism. Otherwise you get ambiguity."
One idea is to explicitly anaphorically refer back to the construct containing the now-unbound variable and talk about its participants, as proposed by Hoemāı and by Hoaqgīo.
"If you have a pet, then the pet in the previously mentioned state of affairs does XYZ." — Hoemaı
"Tu rảı güo kâqgaı jí sa kủsera bı … the one who participates in the guo by seeing something, likes the thing that participates in the guo by being seen." — Hoaqgīo
- Donkey sentence on Wikipedia
- All about poi and noi on the Lojban wiki. The PA da noi section demonstrates a similar "surprising ∀ quantifier".