This essay discusses the question of whether the physical parts of a substance per se can still be substances after being separated from whole substances. This paper finds that within his corpus Aristotle gives two contrary answers to that question. To avoid this inconsistency, this essay claims that Aristotle puts forward dual models of analysis when it comes to substances, namely the Artifact Model and the Living Being Model. According to the Artifact Model, the physical parts of a substance per se are still substances after being separated from the whole substance; but according to the Living Being Model, the physical parts of a substance per se are not substances after separation. In addition, this paper also holds that there is a kind of evolutionary relationship of research methodology between the dual models, i.e. the dual models correspond to what is “better known to us” to what is “better known by nature.”