A 3 year old can intuit it from the context of their learned experience but could you defend the veracity of the claim 1+1=2 from your intellectual superior? The time during which the Authors wrote this book was a very fraught time in Mathematics. They were formalizing the maths of the past 300 years, since the rigor was lacking in many of the brilliant ideas and concepts such as Bertrand's paradox, and the breaking of set theory, worried many mathematicians at the time.