In order for any concept to have meaning, it must be related to sense perception. The 12 categories, or "a priori" concepts, are related to phenomenal appearances through schemata. Each category has a schema. It is a connection through time between the category, which is an "a priori" concept of the understanding, and a phenomenal "a posteriori" appearance. These schemata are needed to link the pure category to sensed phenomenal appearances because the categories are, as Kant says, heterogeneous with sense intuition. Categories and sensed phenomena, however, do share one characteristic: time. Succession is the form of sense impressions and also of the Category of causality. Therefore, time can be said to be the schema of Categories or pure concepts of the understanding.