Who came up with the idea of using variables in mathematics to represent unknown numbers and other unknown entities? It must have been a big conceptual revolution, the idea of variables. Prior to that time, people could only think of specific numbers and equations, like for example the number 3, or the equation 2+3=5. But who was the first to come up with the idea of variables to represent unknown numbers and also make sweeping statement about all numbers? For example, the infinitely many facts that 1+0=0+1, 2+3=3+2, 4+7=7+4, etc. can be summarized by using variables to say: "For all x and y, x+y=y+x". But someone has to have been the first to come up the idea of variables. Who was that person, and do we even have a historical record of the writings of that person?