We didn't define anything. We just used our intuitive understanding of integers and addition. I assume this makes what we were doing uselessly vague?
Your answer makes me think that integers and addition can be defined in different ways. Do different theories exist depending on how we define...
So there is a proof that the sum of any two even numbers is an even number.
2k + 2l = 2(k +l)
We have written the sum as 2 times an integer. Therefore the sum of any two even numbers is an even number.
An essential part of this proof is that k + l is an integer. How do we know this? Is it an...