I don't think it is equally valid.

If GPT-4 was asked to enumerate all possible logical or literal interpretations of the sentence, or to analyse the most literal meaning, perhaps it would have included the second interpretation. But it's tasked with explaining the meaning in the way it is intended to be understood by a regular English speaker.

In natural language, at least in English, some meaning is implied by the absence of words or selection among conventions, when choices are available. The term for this type of non-logical implication is implicature. The Wikipedia article has many examples: https://en.wikipedia.org/wiki/Implicature. This is what GPT-4 is referring to by "The sentence implies".

So, I think most native English speakers wishing to convey the second meaning would not say "I'm afraid the cat has been hit by a car". They would say, for example, "I'm concerned [or worried] the cat may have been hit by a car".

Even if they understand very well what literal, logical interpretations are possible, they assume pragmatically that using "I'm afraid that" along with the absence of "may" would be interpreted by native English listeners as the first interpretation, so the speaker knows to modify the phrasing if they want to avoid that outcome.

This is neatly complemented by the native listeners, who assume native speakers would modify the phrasing if they wanted the second interpretation, so when hearing "I'm afraid the cat has been hit be a car" it's reasonably safe to assume the speaker intends the first interpretation. The speaker assumes the listener knows that, closing the loop.

A type of social contract, if you will. It's part of the language too.

Because implicature interpretations and choices depend so much on probability-matrix evaluations over word choices that are not used but could have been in principle, it seems likely to be the sort of thing GPTs would excel at, compared with old-school language models trying to parse sentences into separable logical components.

As a native English speaker, I disagree.

I would view using that phrase to tell someone that their cat as been hit by a car as being oddly callous and uncaring and therefore I would be much more likely to interpret that phrase as expressing literal fear about an unknown fate. That interpretation is also much more statistically likely as people worry about missing cats more often than they are told they are injured.

In practice, completely context free disambiguation is rare so when doing a thought experiment around one, it is easy to accidentally assume some level of context without realizing.

Well, I'm autistic, but I'm a native speaker and I find both interpretations equally valid. In fact the latter interpretation was where my mind went first.

The key is that if this were a spoken sentence, tone and pace would disambiguate the meaning. Those are lost when writing. Ironically, that's a bit of ambiguity in the prompt which nobody has caught onto yet. The prompt implies that the sentence is spoken aloud, but it's actually being written which creates additional ambiguity. In a written context, someone might be more careful about their word choice (or might not) - which might include not writing that phrase at all because it isn't precise enough...

I took me until you pointed it out to see the second meaning of the sentence. Am I an LLM?

I had a long response written out about it being a general use tool and long answers being less useful, but then I realized...

You aren't an LLM, but you also aren't an English professor, and it was asked to pretend to be that. I would imagine it would be better at pretending to be a professor in a random subject than any random person would. (Assuming they aren't already that kind of professor.)

An LLM would've done a better job with this sentence^