How do we identify a substance?
Since substance is being per se, something that exists in and through itself, it must be something that is a particular being, whole in itself and distinct from all other particular beings. It cannot exist in and through another being or as a part or principle of another being.
Any particular thing that exists in this way is a substance in the primary sense (first substance). And any reference to the genus or species of the particular thing is a reference to substance in the secondary sense (second substance).