 Four main strata of reality are distinguished by Hartmann: the inanimate, the biological, the psychological or mental, and the spiritual. This last includes all historical realities (history, language, customs, law, art, etc.). The underlying observation is that the structure and the laws of history and other spiritual processes are different from the structure and laws of, say, inanimate beings, but the former are not in any way less real than the latter (A20a). The same applies to the other strata as well: biological and psychological processes are as real as any other process, and they have their own specific groups of categories. Ontology must be pluralistic.