I was reading Dirac's "The Principles of Quantum Mechanics" and on page 7 he makes the following statement:
"Like the fundamental concepts (proximity, identity) which everyone must learn on his arrival into the world, the newer concepts of physics can be mastered only by familiarity with their properties and uses."
I take "identity" to mean the vague collection of concepts such as "identity functions/operators", "identities between equations" etc.
But I cannot form any reasonable notion of what "proximity" could mean, even vaguely. My best is "how close things are" and that means almost nothing in this context.