The use of "physical" in physicalism is a philosophical concept and can be distinguished from alternative definitions found in the literature (e.g. Karl Popper defined a physical proposition to be one which can at least in theory be denied by observation). A "physical property", in this context, may be a metaphysical or logical combination of properties which are physical in the ordinary sense. It is common to express the notion of "metaphysical or logical combination of properties" using the notion of supervenience: A property "A" is said to supervene on a property "B" if any change in "A" necessarily implies a change in "B". Since any change in a combination of properties must consist of a change in at least one component property, we see that the combination does indeed supervene on the individual properties. The point of this extension is that physicalists usually suppose the existence of various abstract concepts which are non-physical in the ordinary sense of the word; so physicalism cannot be defined in a way that denies the existence of these abstractions. Also, physicalism defined in terms of supervenience does not entail that all properties in the actual world are type identical to physical properties. It is, therefore, compatible with multiple realizability.