وقدم العناصر جواهرها وصورها، لكن بالنوع
And the eternity of the elements, their substances and forms, but with respect to kind.
Classical natural philosophy counted fire, air, water, and earth as the four elements. In this account individual elemental bodies may change while their kinds, or the succession of those kinds, are claimed to be beginningless.
Individual eternity would mean that this particular entity has never begun; generic eternity claims a kind has always had some instance, even though particular instances come and go. The replacement of individuals does not by itself prove the beginninglessness of the series.