# Why is violating "the only x and y principle" a problem in personal identity theory?

The only x and y principle states that the relation between "x=y" only depends on facts about x & y. So if we say something like "Bob is this particular human being" then this relationship is only determined by facts about "Bob" and facts about "this particular human being". Many people seem to say that violating this principle is a problem for personal identity issues. But I really don't see how this is an issue. For instance, facts about "Bob" are contingent on facts about "Bob's mother" who named him. What's the main problem about violating this?

• I am not sure bringing his mother into it is violating the condition. A 'fact' can involve a whole chain of proof and any number of constants -- as long as it has only one parameter, it is still 'about' the thing that fills that slot.
– user9166
Commented Dec 14, 2014 at 21:20
• It's the relation of identity between `x` and `y` that only depends on facts about `x` and `y`. Other facts about `x` and `y`, except identity, may depend on `w`, `z`, etc.
– n.r.
Commented Jan 26, 2017 at 12:16
• I think you're confusing "Bob" with Bob in your example. Commented Mar 17, 2018 at 22:44