Aristotle is asking why the heaven revolves in one direction rather than the other. His answer is based on his earlier proof that the Universe has a top and a bottom, a right and a left. The upper region, as the place of divinity, is prior or superior to the lower; so upward motion is prior to downward motion. Right is similarly prior and superior to left. The present problem can be solved by supposing that the world has also a front and a back, and hence a forward motion, which will be prior and superior to backward motion. Since Nature always follows the best course, the reason for the direction of the heaven's revolution will be that that direction is forward and therefore better.