 This line of thought proved to be a slippery slope, and, as so often, the downward slide was led by William of Ockham. On the one hand, Ockham was committed to hylomorphism, in that a substance is composed of form and matter. But he reverses the priorities of Aquinas and Scotus by saying that the parts are actual in their own right and do not derive their actuality from the whole: rather, the whole is nothing but the sum of its parts. This line of thought opened the way to atomism and to treating the unity of wholes as a matter of convention or degree. Buridan, who represents a further development in this direction, says