In philosophy, non-essentialism is the belief that, any given entity or subject, can not be propositionally defined in terms of specified values or characteristics which that entity must have in order to be defined as that entity. For example, humanists have an idea of what the essence of being human is. There are specific traits which make something human. This is an essentialist theory. A non-essentialist theory of humanity, would believe that there are no essences or traits that make something human, for example Sartre's theory of existence preceding essence. This view is somewhat problematic however as an entity not defined by any specific values or characteristics may lack any meaningful existence to an observer. Since entities in the real world are defined by their observers in some way in terms of characteristics it would be impossible for a non-essential entity to be found in the real world, though this does not imply that they could not exist in the real world.