[teeja]

The sentence subject is one thing, the sentence <i>topic</i> might be quite another.

Consider sentences like: "He joined the not-yet-famous Liverpool band in early 1958."

To many human beings the topic is quickly obvious. Parsing won't do the trick.

[run4yourlives]

The only reason that sentence is "obvious" to many people is because we have a reference to a famous band from Liverpool that got its start in the late 50's/early 60's that is already embedded in our brain's library of facts.

Removed from that context human beings see that sentence as equally meaningless as a parser, because it is. I'd imagine many young people (who don't have the "correct" reference points) wouldn't have a clue that the sentence is about George Harrison.

In order to properly handle this sentence one would need the same external reference that your brain has. Without the reference the sentence can be discarded as incomplete, since that's what a human would do too.

[unhammer]

You don't necessarily need knowledge to distinguish a topic from a subject though. It's a grammatical distinction. See

http://en.wikipedia.org/wiki/Topic%E2%80%93comment#Definitio... e.g. in the sentence (3) As for the little girl, the dog bit her "the dog" is the subject NP, "the little girl" is the topic. That toy example is perfectly parsable without semantics or even probabilities (though take any real-world sentence and I'm betting you'll need more than just grammar).

[teeja]

Thank you for elaborating my point. But a machine don't need an enormous library of facts to find the topic, just a small one. The words 'he', 'band', 'Liverpool' and '1958' are enough to get very precise ... but not because of their grammatical position.

[yolesaber]

This is why systems such as MIT's ConceptNet exist. NLTK and other NLP systems can provide the grammatical breakdown and ConceptNet can fill in the semantic / contextual gaps.

[grecy]

Ask anyone under ~30, they won't know what you're talking about.

[teeja]

OK just for you: "His 4th album was the second best-selling of the 2000s." The subject of the sentence is "his".

Point remains. Without a context you're lost.

[unlucky]

My existence proves this sentence wrong.

[cs02rm0]

Googling will though.