§ 21a. This "Prolegomena" is a critique of the understanding and it discusses the form and content of experience. It is not an empirical psychology that is concerned with the origin of experience. Experience consists of sense perceptions, judgments of perception, and judgments of experience. A judgment of experience includes what experience in general contains. This kind of judgment results when a sense perception and a judgment of perception are unified by a concept that makes the judgment necessary and valid for all perceivers.