I am 100% reading too much into this, but I am curious which of the properties provided by Spivak allow one to justify a specific argument. For reference/context, the properties are:
P1: If a, b, and c are any numbers, then
$$a +(b + c) = (a + b) +c$$
P2: If a is any number, then
$$a + 0 = 0 +...