Identify a concept from A Separate Peace and come up with a borderline case and an invented case.
I am practicing using a conceptual analysis model (John Wilson) to develop a thesis. One concept I will use is 'friendship". I can't think of another one.
I am guessing that what you want in response to this question is a treatment of other themes in this novel. You have already correctly identified one theme of friendship, which is an extremely good theme to examine. I might suggest at this stage an examination of the theme of war and its treatment in the novel. I will develop this theme for you here and then this will hopefully allow you to examine friendship and its presentation in the novel as well.
I think the crucial part of the text to examine with regard to the theme of war is actually the end of the last chapter. Consider what Gene confesses about his involvement in the war:
I never killed anybody and I never developed an intense level of hatred for the enemy. Because my war ended before I ever put on a uniform; I was on active duty all my time at school; I killed my enemy there.
This quote clearly points towards an expanded definition of what we consider to be "war" - Gene is pointing towards the fact that in some senses our lives can be considered to be lived in wartime, whether or not there is an actual war going on. In many senses, the war in A Separate Peace provides a visual backdrop to what is going on in the hearts and minds of the characters, as Gene goes on to state:
Other people experienced this fearful shock somewhere, this sighting of the enemy, and so began an obsessive labor of defense, began to parry the menace they saw facing them by developing a particular frame of mind...
However, as the novel concludes, this state of mind is ultimately futile and perhaps even damaging to the person themselves as shown by the other characters and their response to war:
All of them, all except Phineas, constructed at infinite cost to themselves these Maginot Lines against this enemy they thought they saw across the frontier, this enemy who never attacked that way - if he ever attacked at all; if he was indeed the enemy.
Perhaps, Gene suggests, as he has learned, that the real enemy isn't who we suppose the real enemy is, but is in fact within you - that is the real "war" that is carried on and that we fight - and so often with such tragic results, as the death of Finny shows.