Concerning the "nous" that is the source of understanding of individuals, Plato is widely understood to have used ideas from Parmenides in addition to Anaxagoras. Like Parmenides, Plato argued that relying on sense perception can never lead to true knowledge, only opinion. Instead, Plato's more philosophical characters argue that "nous" must somehow perceive truth directly in the ways gods and daimons perceive. What our mind sees directly in order to really understand things must not be the constantly changing material things, but unchanging entities that exist in a different way, the so-called "forms" or "ideas". However he knew that contemporary philosophers often argued (as in modern science) that "nous" and perception are just two aspects of one physical activity, and that perception is the source of knowledge and understanding (not the other way around).