 (b) Aristotle also offers (1050b6–1051a2) an “even stricter” argument for his claim that actuality is prior in substance to potentiality. A potentiality is for either of a pair of opposites; so anything that is capable of being is also capable of not being. What is capable of not being might possibly not be, and what might possibly not be is perishable. Hence anything with the mere potentiality to be is perishable. What is eternal is imperishable, and so nothing that is eternal can exist only potentially—what is eternal must be fully actual. But the eternal is prior in substance to the perishable. For the eternal can exist without the perishable, but not conversely, and that is what priority in substance amounts to (cf. Δ.11, 1019a2). So what is actual is prior in substance to what is potential.