Abstract: In this paper I wish to ask whether philosophers have good grounds for elaborating rhetorics of science. By doing so they might seem to deny the distinction between the theoretical intellect and the practical intellect, which traditionally have reigned over scientific discourse and rhetorical discourse, respectively. I shall suggest that philosophers of rhetoric do indeed have a warrant for developing their rhetorics of science. We shall assume with Aristotle that we may distinguish the theoretical from the practical intellect by distinguishing objects which cannot be other than they are from objects which can be other than they are. What we shall find is that a stalwart of British empiricism, John Stuart Mill, develops a philosophy of science concerned with objects which can be other than they are. Mill thus provides us with an ontological justification for our new rhetorics of science.