Full Idea
Examples of unnatural movements are something earthy moving upwards and fire moving downwards. …When they are moved unnaturally it is obvious what they are moved by, but this is not obvious in the case of their natural movements.
Gist of Idea
When there is unnatural movement (e.g. fire going downwards) the cause is obvious
Source
Aristotle (Physics [c.337 BCE], 254b21)
Book Reference
Aristotle: 'Physics', ed/tr. Waterfield,Robin [OUP 1996], p.196
A Reaction
Aristotle always struggles when he tries to give an account of 'unnatural' events. It is hard to see how it is unnatural when a wind blows a flame down, or volcanoes blow earth up. There is hardly a natural distinction of causes which are 'obvious'.