The expression that one plus one equals two comes from a basic mathematical principle, that is, the law of addition of natural numbers. In piano's axiomatic system, natural numbers are defined as an infinite sequence from the beginning, and each number after it is the previous number plus one. When you say "plus one", you are actually doing a simple arithmetic operation, that is, adding a certain number to one. When this number is 1, add 1 and add 1 to get 2.

This result can be verified by counting: if there is an apple and the other apple is taken away, there are always two apples. "One plus one equals two" is also a basic fact in mathematics and the core component of arithmetic and mathematical logic.