þæs is the form for genitive of se and þæt, the masculine's and neuter's singular for "the." The nominative forms are sometimes used on their own as well implying a subject, just as "that" still is today in English.
That (work) is great!
We don't use a genitive form or inflection for "that" anymore. But we could:
Aristotle wrote the collection of books that we now call the "Metaphysics", but he did not give his text any title. Our title comes from Andronicus of Rhodes. Putting together the first compilation of Aristotle's works (about 250 years after Aristotle died), Andronicus used the phrase "meta ta physica" to refer to this text, and we have known it as the "Metaphysics" ever since. It is not clear why Andronicus used this phrase; the standard speculation is that, in his list of Aristotle's works, he had located this text 'after' another text - namely, the one that Aristotle had entitled the "Physics".
Aristotle himself never used the phrase "meta ta physica". He referred to his Metaphysics subject matter in various ways, calling it "wisdom", "first philosophy" (because it concerned first principles), "the science of being" (because that was the subject it concerned), and "theology" (because the gods were preeminently real).
So I think George may miss the point. He is right in observing that the book we now know as Aristotle's Metaphysics concerns a knowledge beyond everyday reality. But Kevin's question about "ton" apparently would turn on speculations as to Andronicus' motives in using his obscure phrase, not Aristotle's. And Andronicus would have been well aware that Aristotle never referred to his subject-matter as 'knowledge beyond everyday reality'. So whatever Andronicus may have meant, it must have been something else.