Aristotle holds that there are four kinds of answers to "why" questions:

Matter (the material cause of a change or movement): The aspect of the change or movement that is determined by the material that composes the moving or changing things. For a table, such might be wood; for a statue, such might be bronze or marble.

Form (the formal cause of a change or movement): A change or movement caused by the arrangement, shape, or appearance of the thing changing or moving. Aristotle says, for example, that the ratio 2:1, and number in general, is the cause of the octave.

Agent (the efficient or moving cause of a change or movement): Consists of things apart from the thing being changed or moved, which interact so as to be an agency of the change or movement. For example, the efficient cause of a table is a carpenter, or a person working as one, and according to Aristotle the efficient cause of a boy is a father.

End or purpose (the final cause of a change or movement): A change or movement for the sake of a thing to be what it is. For a seed, it might be an adult plant; for a sailboat, it might be sailing; for a ball at the top of a ramp, it might be coming to rest at the bottom.