Protagoras was a Greek guy who sounded a lot like today's adolescent: "Man is the measure of all things," meaning that what you feel, perceive, and want the world to be is what it actually is.
Aristotle objected for most of the same reasons you may object: "Protagoras said that man is the measure of all things, meaning simply and solely that what appears to each man assuredly also is. If this is so, it follows that the same thing both is and is not, and is both bad and good, and whatever else is asserted in contrary statements, since often a particular thing appears good (or beautiful) to some and the opposite to others; the criterion is what appears to each individual." (Metaphysics 1062b13)
That is, Aristotle is saying that if Protagoras (and today's modern relativist) is right then we cannot know whether something is good or bad, true or false for anyone except ourselves. God may be good for me and bad for you--we're equally right.
If that sits uneasily in your stomach, you will love getting to know Aristotle and his friends...