§ 23. Pure "a priori" principles of possible experience bring unter mere phenomenal appearances under pure concepts of the understanding. This makes the empirical judgment valid in reference to an external object. These principles are universal laws of nature which are known before any experience. This solves the second question "How is the pure science of nature possible?". A logical system consists of the forms of all judgments in general. A transcendental system is made up of the pure concepts which are the conditions of all synthetical, necessary judgments. A physical system, which is a universal and pure science of nature, contains pure principles of all possible experience.