In logic and metaphysics, properties are attributes or characteristics, universals that may belong to one or many things at the same time. Properties have extensions, that is, a class of things to which they belong, which may be empty. But different properties (having a heart, having a liver) may characterize the same extensions. Much metaphysics circles around whether two apparently different properties can properly be regarded as one. Other parts tackle the question of the difference between the essential and the accidental properties of things. The problem of universals is an attempt to understand or even to abolish the distinction between particular things and their properties.