To define himself further, Descartes turns to the example of wax. He determines that wax isn't wax because of its color, texture or shape, as all of these things can change and the substance still be wax. He believes that wax is perceived "by the intellect alone." Therefore, he distinguishes between "ordinary perception" and "judgment". When one understands the mathematical principles of the substance, such as its expansion under heat, figure and motion, the knowledge of the wax can be "clear and distinct".