When you write “1+1” you may mean two things: “the result of doing the adding operation to 1 and 1“, and “the successor of 1”. It just happens that we use “+1” to denote both of those. The fact that successor(1) = add(1,1) isn’t completely trivial.
Principia Mathematica, though, takes a different line. IIRC, in PM “2” means something like “the property a set has when it has exactly two elements” (i.e., when it has an element a and an element b, and a=b is false, and for any element x we have either x=a or x=b) and similarly for “1” (with all sorts of complications because of the hierarchy of kinda-sorta-types PM uses to try to avoid Russell-style paradoxes). And “m+n” means something like “the property a set has when it it is the union of two disjoint subsets, one of which has m and the other of which has n”. Proving 1+1=2 is more cumbersome then. And PM begins from a very early point, devoting quite a lot of space to introducing propositional calculus and predicate calculus (in an early, somewhat clunky form).
When you write “1+1” you may mean two things: “the result of doing the adding operation to 1 and 1“, and “the successor of 1”. It just happens that we use “+1” to denote both of those. The fact that successor(1) = add(1,1) isn’t completely trivial.
Principia Mathematica, though, takes a different line. IIRC, in PM “2” means something like “the property a set has when it has exactly two elements” (i.e., when it has an element a and an element b, and a=b is false, and for any element x we have either x=a or x=b) and similarly for “1” (with all sorts of complications because of the hierarchy of kinda-sorta-types PM uses to try to avoid Russell-style paradoxes). And “m+n” means something like “the property a set has when it it is the union of two disjoint subsets, one of which has m and the other of which has n”. Proving 1+1=2 is more cumbersome then. And PM begins from a very early point, devoting quite a lot of space to introducing propositional calculus and predicate calculus (in an early, somewhat clunky form).