Aristotle rejected Plato’s claim that only the forms are real and that there is another world of forms outside of the world that we perceive in ordinary life. But he agreed with Plato that knowledge must have certainty. Therefore, his main philosophical task was to describe what made objects real in this world and explain how we can have certain knowledge about them. He also developed a system of logic, or rules of thought, that would guarantee certainty if one began with premises that were certain.