Contrary to readings that consider Physics I 5 as doxographical, this chapter argues that it is the first step of a constructive inquiry whose conclusions, paradoxical as they may seem, are never dismissed later. Its main thesis is that the contraries are principles of coming-to-be and passing-away, but it also endorses the stronger thesis that the principles are contraries. Although it appeals to doxographical considerations, its main section is an inductive argument which heralds the concept of privation, without mentioning it explicitly. This chapter studies the relationship between the opposition of contraries and that of possession and privation, as well as the way Aristotle reduces the intermediates to the contraries. Finally, it shows how the device of sustoikhia (‘series’) allows Aristotle to identify the opposition between possession and privation as the first pair of contraries and to organize the positions of his forerunners in respect of their proximity to the truth.