I'm re-reading Greene's The Fabric of the Cosmos. He describes Newton as explaining his (Newton's, not Greene's!) thought experiment of the bucket by the existence of absolute space: something with respect to which rotating objects rotate, even in seemingly empty space. Did Newton ever discuss why rotational motion was absolute, but constant velocity motion was relative? Or did he in fact think all motion was absolute? I guess that would mean he would have predicted that the Michelson-Morley would have found an ether wind.