@Waz: yes, the good, pure, it's a whole concept, not just a word.
And yes I think it's a noun (i'm not good at this stuff).
If you wanted to say someone or something is good you'd most probably use "kalos" (even though in ancient greek you could use "agathos" too i guess).
you smart bastards. I would fly to athens, only I have no money, no direction in life, no spare time, no gumption and absolutely no knowledge of greek philosophy Plus my A level exams begin around that time!