Cons
Define cons. A cons cell holds an element and a pointer to the rest of the structure. That’s exactly what pair does: head is the element, tail is the rest.
With cons you can chain values together:
cons(1, cons(2, cons(3, ???)))The recursion through the chain works. head gives you the current element, tail gives you the rest, and you can keep going. But look at that ??? at the end. What goes there?
You could put 0. But then a function walking the chain can’t distinguish “the list ends here” from “the next element is zero.” You could put false. Same problem. Any value you pick as the terminator could also be a legitimate element.
This is the tension: you can build a chain and walk it, but you can’t write the base case. A recursive function like length needs to ask “is this the end?” and there’s no reliable answer yet.
You need a value that is distinct from every number, every boolean, and every function. A value whose only purpose is to say “nothing here.”