Some systems only work because we agree they do
This is something that has always fascinated me: how some things in society are only there because of some ingrained common agreement, not for a physical reason. This includes things like why women wear skirts and men don’t, and why we are ashamed when our hair goes grey even though it happens to everybody, whether at age 25 or 75.
But the most fascinating arbitrary system is economics. All of it – insurance, investments, trust funds, exchange rates, shares, interest rates, etc. – only works because society as a whole has agreed to it, so long ago that nobody thinks about it anymore. It now seems inherent, even though nothing about it is actually inherent.
This probably explains why economics is a social science, like psychology or criminology, as it is completely dependent on humanity and society to function.