True Nature is the fundamental substance or body of all things. It manifests infinite phenomena according to causes and conditions. Such infinite phenomena make up the Ten Realms, which include Four Realms of Sages and Six Unenlightened Realms. The Truth at the Buddha’s level is the Buddha Nature. From the human’s perspective, the truth becomes human nature. The fundamental substance of the human mind is the ultimate nature—the Buddha Nature—which does not change; but the human mind does change in accordance with causes and conditions.