Aristotle on Causality, The Four Causes

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To be able to give a rational account of constant change in the realm of natural beings and consequently to lay ground for physics as an explanatory potent science Aristotle introduces a scheme of causal relations.

Nature itself is a principle and a cause of change. But we speak about the cause with regard to four different points of reference each pointing to one aspect of the more general question "why something is".

To ask "why something is" means to identify main factors in the process of potentiality realization. Aristotle explicates this question in a fourfold way:

1. Out of what has a thing come?

Answer obtained by identifying: The Material Cause:

The material cause points to "that from which, as a constituent, an object comes into being." (For instance, the bronze of a statue.)

2. What is it?

Answer obtained by identifying: The Formal Cause:

The formal cause embodies the essential nature (all essential attributes) and represents the model or archetype of the outcome; conceptually it is expressed in the definition (logos). (It is the idea of the statue as present in artist's head.)

3. By means of what is it?

Answer obtained by identifying: The Efficient Cause:

The efficient cause is "the source of the change or rest"; it is the moving cause: "what makes of what is made and what changes of what is changed" (the sculptor who makes the statue).

4. For the sake of what is it?

Answer obtained by identifying: The Final Cause:

The final cause states "that for the sake of which" a thing is done, or, in other words, it explicates something's end (the final shape or the effect on the audience which admires the statue).

Note: Although Aristotle himself holds all these four causes responsible for any real change and movement (aitia in Greek are those things that are "guilty" or responsible for something), they are rather demarcation points of change as revealed in our language than true causes (with a possible exception of the efficient cause, which is nowadays considered to be the only real cause out of the four). In difference to the modern concept of causation, which always implies a sequence of two events, Aristotle envisions causation as a single event of double actualization: agent's potential to effectuate something and patient's potential to sustain that change.

http://www.uri.edu/personal/szunjic/philos/physics.htm