I said that a primary substance conceived of as an 'instance of a nature' might be conceived of, also, as the substance as distinct from its accidents.
Would Aristotle allow such a conception? An argument that he would is: he allows it for items in the other categories; therefore, he would allow it also for substance.
That he allows it for items in the other categories is shown in a curious passage from his treatment of relatives. In this passage, he argues that a relative always has a correlative, so long as the relative and correlative are given correctly:
Again, if that in relation to which a thing is spoken of is properly given, when all the other things that are accidental are stripped off and that alone is left to which it was properly given as related, it will always be spoken of in relation to that. For example, if a slave is spoken of in relation to a master, then, when everything accidental to a master is stripped off--like being a biped, capable of knowledge, a man--and there is left only its being a master, a slave will always be spoken of in relation to that. For a slave is called slave of a master. On the other hand, if that in relation to which a thing is spoken of is not properly given, then, when the other things are stripped off and that alone is left to which it was given as related, it will not be spoken of in relation to that. Suppose a slave is given as of a man and a wing as of a bird, and strip off from man his being a master; a slave will no longer be spoken of in relation to a man, for if there is no master there is no slave either. Similarly, strip off from bird its being winged; a wing will no longer be a relative, for if there is nothing winged neither will there be a wing of anything 7a31-b9, Ackrill).Note that as being a man is accidental to being a master, so is being a master accidental to being a man. So we might strip off being a master from being a man, in just the same way as Aristotle here contemplates stripping off being a man from being a master. And then the same thing might be done with the other accidents.
e1ti e0a_n me\n oi0kei/wj a)podedome/non h|} pro_j o4 le/getai, pa&ntwn periairoume/nwn tw~n a1llwn o3sa sumbebhko&ta e0sti/n, kataleipome/nou de\ tou&tou mo&nou pro_j o4 a)pedo&qh oi0kei/wj, a)ei\ pro_j au)to_ r(hqh&setai: oi[on ei0 o( dou~loj pro_j despo&thn le/getai, periairoume/nwn a(pa&ntwn o3sa sumbebhko&ta e0sti\ tw|~ despo&th|, oi[on to_ di/podi ei]nai, to_ e0pisth&mhj dektikw|~, to_ a)nqrw&pw|, kataleipome/nou de\ mo&nou tou~ despo&thn ei]nai, a)ei\ o( dou~loj pro_j au)to_ r(hqh&setai: o( ga_r dou~loj despo&tou dou~loj legetai. e0a_n de/ ge mh_ oi0kei/wj a)podoqh|~ pro_j o3 pote le/getai, periairoume/nwn me\n tw~n a1llwn kataleipome/nou de\ mo&nou tou~ pro_j o4 a)pedo&qh, ou) r(hqh&setai pro_j au)to&: a)podedo&sqw ga_r o( dou~loj a)nqrw&pou kai\ to_ ptero_n o1rniqoj, kai\ perih|rh&sqw tou~ a)nqrw&pou to_ despo&th| au)tw|~ ei]nai: ou) ga_r e1ti o( dou~loj pro_j a1nqrwpon r(hqh&setai, —mh_ ga_r o1ntoj despo&tou ou)de\ dou~lo&j e0stin:— w(sau&twj de\ kai\ tou~ o1rniqoj perih|rh&sqw to_ pterwtw|~ ei]nai: ou) ga_r e1ti e1stai to_ ptero_n tw~n pro&j ti: mh_ ga_r o1ntoj pterwtou~ ou)de\ ptero_n e1stai tino&j.
Presumably, too, in the above passage 'man' indicates the primary substance. To strip away man from being a master just is to strip away the primary substance. But clearly if a man can be stripped away from a relative, so the relative and other accidents can be stripped away from a man.
Thus the primary substance is not the individual, but the individual with the accidents stripped away.