This quotation comes from Thomas Jefferson, who goes on to say that "action will delineate and define you". Introspection helps you understand yourself, but only if you do something to examine.
Think about the last time you saw a movie and you liked a character in it. Did you "bond" with the character because they said they were good or because another character thought so? Probably not, because we tend to appreciate people through their actions. We like what people do a lot more than what they say, even if they have a silver tongue. The same goes for you. Your actions determine who you are and how you view yourself. When you do nothing, you question who you are. When you do something well, you get an answer.
Do you want to know who you are? [BrainyQuote]