Underfilling
To underfill a verb is to provide it with fewer arguments than there are slots in its definition.
Using a transitive verb as though it were intransitive (Huogaı jí da.) or using an intransitive verb as though it were nullary (Loe ꝡo!) are both examples of underfilling.
The common consensus is that this is allowed, but it's not so clear what exactly it means, and how it comes to mean that.
In logic or mathematics, there's no sensible way to "underfill" a relation. "Twelve is divisible by three" makes sense, but "Twelve is divisible" does not. So when we turn "I hear it" into "I hear", what is really going on?
Theories of underfilling
Verb families
One explanation is that, for example, huogaı is actually a "verb family" of three verbs in a trenchcoat:
huogaı2: ___ hears ___.
huogaı1: ___ hears.
huogaı0: Some hearing happens.
Depending on how many arguments we provide, huogaı selects a completely different lexical item.
This is tidy, but it doesn't gives us a predictable rule for what the intransitive version of a transitive verb means. In English, "I know" means "I know it" but "I eat" means "I eat something". Should this distinction be mindlessly carried over into Toaq if we define dua1 and chuq1 as above?
Implicit arguments
Another explanation is that when we underfill huogaı, the remaining slots are filled with some implicit argument. But which? All of sá raı, báq raı, ké raı, and a "vague definite reference" / pronoun seem to make sense in different situations.
Disallowing underfilling
An extreme idea is to ban sentences like Huogaı jí, forcing the speaker to say something explicit and specific like Huogaı jí sá or Huogaı jí hóq. This is semantically watertight but annoying.
Other contexts
Determiners
Even saying something like sá chuq invokes our theory of underfilling, as chuq does not have an object. So, does it mean "someone who eats it", or "someone who eats something"? Or does it simply involve the intransitive chuq1? For words with two slots, this can be solved by using object incorporation.
Subclauses
Underfilling is typically not possible in a subclause:
*Laheq, ꝡä moı jí, ꝡä jıq jí.
(Attempted:) That I think, entails that I exist.
The verb moı is transitive. Given how self-termination works / because subclauses are "greedy", ꝡä jıq jí ends up being the object of moı, not of laheq. Thus, this sentence actually means "That I think about that I exist, entails (it/something)."
We can use the prefix hạo, which turns verbs intransitive by applying whatever theory of underfilling we subscribe to:
Laheq, ꝡä hạomoı jí, ꝡä jıq jí.
That I think, entails that I exist.