In philosophy, supervenience is an ontological relation that is used to describe cases where (roughly speaking) the upper-level properties of a system are determined by its lower level properties. Some philosophers hold that the world is structured into a kind of hierarchy of properties, where the higher level properties supervene on the lower level properties. According to this type of view, social properties supervene on psychological properties, psychological properties supervene on biological properties, biological properties supervene on chemical properties, etc. That is, the chemical properties of the world determine a distribution of biological properties, those biological properties determine a distribution of psychological properties, and so forth. So, for example, mind-body supervenience holds that "every mental phenomenon must be grounded in, or anchored to, some underlying physical base (presumably a neural state). This means that mental states can occur only in systems that can have physical properties; namely physical systems."