In philosophy, supervenience is an ontological relation that is used to describe cases where (roughly speaking) the lower-level properties of a system determine its higher 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."It is useful to know both when supervenience does and does not obtain. For example, the value of a piece of paper money does not supervene on the micro-features of the paper it's made out of, because the value of the money is not just determined by internal features of the paper, but also by a broader distribution of social facts and institutions. The paper alone does not determine the value of the money, so supervenience does not obtain between the value and the paper (though it does obtain between the value of the money and some larger part of the world which includes the relevant social institutions). Some philosophical problems can be stated in terms of questions about supervenience. For example: do mental properties supervene on neural properties? (Externalists in the philosophy of mind say "no", internalists say "yes.") Do the aesthetic properties of an art object supervene on physical properties of that object?Though it is intuitively clear what supervenience is, formally specifying what it means involves a complex technical apparatus and a family of related but subtly different definitions. Most definitions involve comparisons of objects for indiscernibility. According to one standard definition, a set of properties A (e.g. mental properties) supervenes on a set of properties B (e.g. neural properties), if and only if any two objects x and y which share all properties in B (are "B-indiscernible") must also share all properties in A (are "A-indiscernible"). The intuitive idea is that if you could make a physical copy of a person, you'd also be making a psychological copy of that person. The reverse does not hold: two people could be in the same mental state, but that mental state could be supported by different brain states (the same mental state could be "multiply realizable" by different brain states). The properties in B are called the base properties (or sometimes subjacent or subvenient properties), and the properties in A are called the supervenient properties.Supervenience has traditionally been used to describe relationships between sets of properties in a manner which does not imply a strong reductive relationship. For example, many hold that economic properties supervene on physical properties, in that if two worlds were exactly the same physically, they would also be the same economically. However, this does not entail that economics can be reduced in any straightforward way to physics. Thus, supervenience allows one to hold that "high-level phenomena" (like those of economics, psychology, or aesthetics) depend, ultimately, on physical substance, without assuming that one can study those high-level phenomena using means appropriate to physics.