Matter, the ever-persistent, which assumes different conditions in change on account of the presence of different qualities, and the forms which animate it, constitute the world. But qualities, Aristotle holds, are real existence. All things, in Aristotle, are compelled by an inner necessity to outgrow themselves and realise their purpose in a form which exists as the potential in matter. Everything is matter and form at the same time, the higher being the form of the lower and the lower the matter of the higher. There is an evolution of the higher form from the lower, which exists as the matter or potentiality of the higher.
Form is the total force residing in a thing as the very essence of being, doing and becoming.
There is no external mechanical cause in the unfoldment of the actuality of things but the real cause is the internal necessity which works with due reference to the type to which the things belong. Every change in a thing is guided by a purpose and end, a goal which is the actualisation of the higher form. The potential becomes actual at every stage. Matter has a tendency, a desire or a love to realise its form, and here it co-operates with the function of the necessity directed by the great purpose which consists in the realisation of the form. When man works on a material these four causes are visible but in organic nature the instrument of action is identical with the form and the unrealised also is the form, so that only form and matter remain in the end as the only two causes.