"The main concern of philosophy is to question and understand very common ideas that all of us use every day without thinking about them. A historian may ask what happened at some time in the past, but a philosopher will ask, "What is time?" A mathematician may investigate the relations among numbers, but a philosopher will ask, "What is a number?" A physicist will ask what atoms are made of or what explains gravity, but a philosopher will ask how we can know there is anything outside of our own minds. A psychologist may investigate how children learn a language, but a philosopher will ask, "What makes a word mean anything?" Anyone can ask whether it's wrong to sneak into a movie without paying, but a philosopher will ask, "What makes an action right or wrong?""