Thank you for an example with fruit. I am not trying to be thick, but your last paragraph confuses me. My point of reference is, say, lending $1,000 to a sibling who will pay me back within 2 weeks when he gets his paycheck. I gave him $1,000 and he will return me $1,000 so when and how would interest arise here? What axiom am I missing that makes his debt increase? Is it a choice by me to demand it?