Since philosophical thought, including religious beliefs, is documented, humans have tried to explain the natural world in terms of the smallest possible number of fundamental elements. Anaximenes proposed air as such a fundamental element; Tales proposed water. Concerned about the great complexity of the world, Empedocles thought that the elements must be four: air, water, earth, and fire. An immediate consequence of dealing with a limited set of elements implies the possibility to develop a unitary description, or a theoretical framework, for all phenomena.