 Russell’s paradox ultimately stems from the idea that any condition or property may be used to determine a set. For example, the property of being evenly divisible by only itself and the number one distinguishes the set of prime numbers from the set of whole numbers. The property of having mammary glands distinguishes the set of mammals from reptiles, birds and other living organisms. The property of being both square and not square (or any other conjunction of contradictory properties) determines the empty set, and so on.