Aristotle's philosophical works continue many of the same Socratic themes as his teacher Plato. Amongst the new proposals he made was a way of explaining causality, and "nous" is an important part of his explanation. As mentioned above, Plato criticized Anaxagoras' materialism, or understanding that the intellect of nature only set the cosmos in motion, but is no longer seen as the cause of physical events. Aristotle explained that the changes of things can be described in terms of four causes at the same time. Two of these four causes are similar to the materialist understanding: each thing has a material which causes it to be how it is, and some other thing which set in motion or initiated some process of change. But at the same time according to Aristotle each thing is also caused by the natural forms they are tending to become, and the natural ends or aims, which somehow exist in nature as causes, even in cases where human plans and aims are not involved. These latter two causes (the "formal" and "final"), are concepts no longer used in modern science, and encompass the continuous effect of the intelligent ordering principle of nature itself. Aristotle's special description of causality is especially apparent in the natural development of living things. It leads to a method whereby Aristotle analyses causation and motion in terms of the potentialities and actualities of all things, whereby all matter possesses various possibilities or potentialities of form and end, and these possibilities become more fully real as their potential forms become actual or active reality (something they will do on their own, by nature, unless stopped because of other natural things happening). For example, a stone has in its nature the potentiality of falling to the earth and it will do so, and actualize this natural tendency, if nothing is in the way.