From Middle English selfsame, from earlier selve same, equivalent to self +"Ž same or self- +"Ž same. Compare Old High German selbsama (“selfsame, identical"), Danishselvsamme (“selfsame, identical"), English selfsome. More at self, same.
English Wiktionary. Available under CC-BY-SA license.
It follows that we must call this selfsame essence, at once individual and universal, substance - a conclusion, however, which Aristotle never drew in so many words, though he continued always to call essence substance, and definition a knowledge of substance.
Each natural substance is a formal cause, as being what it is; a material cause, as having passive power to be changed; an efficient cause, as having active power to change, by communicating the selfsame essence into different matter so as to produce therein a homogeneous effect in the same species; and a final cause, as an end to be realized.
Sensation is not the reception of the selfsame essence of an external body, but one's perception of one's sentient organism as affected, and especially of its organs resisting one another, e.g.