Great book. Reading it for the second time. There are several "jumps" that are made in their lines of reasoning that certainly make the explanations easier to follow, but also left me wondering "Why or where did that assumption or statement come from?" On page 77 (in my 2009 Da Capo Press version) when working towards setting up the spacetime model, they state: "The length of the hypotenuse must be either s^2 = (ct)^2+x^2 or s^2 = (ct)^2-x^2. There is no other option." So why is the negative version even considered as a option? They try to address this in the paragraph following this, but really don't. They just say it is the only other option. Can anyone explain to me why that is the only other option? Why is not s = ct + x not considered an option? Or any other s = function(ct,x)?