Set theory begins by giving some examples of things that are sets. Then it gives rules in which you can make other sets from the already known sets. Collections of objects that are not sets are called (proper) classes. It is possible to do mathematics using only sets, rather than classes, so that the problems that classes cause in mathematics do not occur.
s ɛ t WORD_BOUNDARY θ iə ɹ i WORD_BOUNDARY b ɪ ɡ ɪ n z WORD_BOUNDARY b aɪ WORD_BOUNDARY ɡ ɪ v ɪ ŋ WORD_BOUNDARY s ʌ m WORD_BOUNDARY ɛ ɡ z æ m p ə l z WORD_BOUNDARY ʌ v WORD_BOUNDARY θ ɪ ŋ z WORD_BOUNDARY ð æ t WORD_BOUNDARY ɑ ɹ WORD_BOUNDARY s ɛ t s WORD_BOUNDARY ð ɛ n WORD_BOUNDARY ɪ t WORD_BOUNDARY ɡ ɪ v z WORD_BOUNDARY ɹ uː l z WORD_BOUNDARY ɪ n w ɪ t̠ʃ WORD_BOUNDARY j uː WORD_BOUNDARY k æ n WORD_BOUNDARY m eɪ k WORD_BOUNDARY ʌ ð ə ɹ WORD_BOUNDARY s ɛ t s WORD_BOUNDARY f ɹ ʌ m ð ɪ WORD_BOUNDARY ɔ l ɹ ɛ d i WORD_BOUNDARY n oʊ n WORD_BOUNDARY s ɛ t s WORD_BOUNDARY k ə l ɛ k ʃ ə n z WORD_BOUNDARY ʌ v WORD_BOUNDARY ɑ b d̠ʒ ɛ k t s WORD_BOUNDARY ð æ t WORD_BOUNDARY ɑ ɹ WORD_BOUNDARY n ɑ t WORD_BOUNDARY s ɛ t s WORD_BOUNDARY ɑ ɹ WORD_BOUNDARY k ɔ l d WORD_BOUNDARY p ɹ ɑ p ə ɹ WORD_BOUNDARY k l æ s ɪ z WORD_BOUNDARY ɪ t WORD_BOUNDARY ɪ z WORD_BOUNDARY p ɑ s ɪ b ə l WORD_BOUNDARY t ə WORD_BOUNDARY d uː WORD_BOUNDARY m æ θ ɪ m æ t ɪ k s WORD_BOUNDARY j uː z ɪ ŋ WORD_BOUNDARY oʊ n l i WORD_BOUNDARY s ɛ t s WORD_BOUNDARY ɹ æ ð ə ɹ WORD_BOUNDARY ð ʌ n WORD_BOUNDARY k l æ s ɪ z WORD_BOUNDARY s oʊ WORD_BOUNDARY ð æ t ð ə WORD_BOUNDARY p ɹ ɑ b l ə m z WORD_BOUNDARY ð æ t WORD_BOUNDARY k l æ s ɪ z WORD_BOUNDARY k ɔ z WORD_BOUNDARY ɪ n WORD_BOUNDARY m æ θ ɪ m æ t ɪ k s WORD_BOUNDARY d uː n ɑ t WORD_BOUNDARY ə k ɜː WORD_BOUNDARY