This is a rather complicated and long-ish read, so I decided to separate the following wall of text into several thematic chunks.
The philosophical bit
Being a philosopher by trade (as if one could trade with that) I excel at very little except (over)thinking. Applying said thinking to “game design” or the question of what gives a game its identity I find myself confronted with a version of the paradox of the heap, also called sorites paradox. There is a positive and a negative version of the paradox. I will only concern myself with the positive version, which is as follows:
(1) A collection of one million grains of sand is a heap.
(2) If a collection of n grains of sand is a heap then so is a collection of n-1 grains.
(3) A collection of one grain of sand is a heap.
Now, oof. What does that have to do with throwing dice and playing ICRPG, D&D or whatever?
If we apply the sorites paradox to TTRPGs and game mechanics, we can reformulate it as such:
(1) A collection of n game mechanics is a specific game (let’s say ICRPG)
(2) If a collection of n game mechanics is ICRPG then so is a collection of n-1 game mechanics.
(3) A collection of one game mechanic is ICRPG.
As far as I can see there’s at least two solutions to the paradox if applied to the example I chose here.
The first solution would be to deny that “ICRPG” is a vague predicate whereas “a heap” is. One could simply say: “Shoo! Away with your logical paradoxes. You are overthinking this.” and declare that ICME p. 5 clearly lists the key system innovations that make ICRPG ICRPG: Turns, Effort Dice, Targets, Clean STATS.
Taking away any of those would make ICRPG not ICRPG anymore. Easy-peasy.
This provokes another question, though: if I took these key system innovations and added other things to them, would there be a point where I would not be playing ICRPG anymore but something else (another “new” game?).
The second solution would be to argue that applying the paradox to the question what number of game mechanics makes ICRPG ICRPG is a purely logical and formal problem that has nothing to do with the real world of gaming.
In other words: the problem arises not because it is a real problem but because the philosopher (in this case: me) has simply misused language. Everybody understands what we mean when we say that we play ICRPG or can at least talk about what we mean whne we say that we play ICRPG. One might even argue that it is not a matter of X game mechanics but rather a matter of a certain mindset. Asking which game mechanics make ICRPG ICRPG would therefore be the wrong question.
The moral bit
So, why all this stuff about paradoxes and game mechanics? What remains in my opinion is not a logical problem but a moral question prompted by the question that arises from the first solution: if one made a game and took the mechanics found in ICRPG would any additional rules ever change the identity of the game? Would any game using mechanics found in ICRPG ever be its own game or would it remain an ICRPG-hack? When will it become its own game and when will it remain a hack?
If I took the key system mechanics (or maybe only some of them), renamed them as to avoid copyright claims and put them in a game that I wrote and then sold: would it really be my own game or am I just taking other people’s hard work to profit from them? Wouldn’t I not only creatively but also morally be obligated to do my own stuff?
The tl;dr bit
Are game mechanics part of a game’s identity? Can you take key mechanics of games you like and put them in your own (commercial) game without feeling like a hack?