 Supervenience relations are covariance relations that have three logical features: they are reflexive, transitive, and non-symmetric. The claim that supervenience is reflexive means that every set of properties supervenes on itself: for any class of properties A, there can be no difference in the A-properties without a difference in the A-properties. The claim that supervenience is transitive means that: if the A-properties supervene on the B-properties, and the B-properties supervene on the C-properties, then the A-properties supervene on the C-properties. The claim that supervenience is non-symmetric means that supervenience is compatible with either symmetry (A supervenes on B and B supervenes on A; as in the case of the ethical and itself) or asymmetry (A supervenes on B but B does not supervene on A; as may be the case between the biological and the microphysical).