I'm watching the video lectures of Financial Theory (ECON 251) by John Geanakoplos, Yale University. In Lesson 5, Chapter 4 at 33:41, Geanakoplos defined inflation as the ratio of prices between two periods. There was no metion of money in his model whatsoever.
On the other hand, macroeconomic theory textbooks usually introduce money first before inflation. Only after incorporating money into macroeconomic models would they start to discuss about inflation. They then would go on to derive the relation between inflation and money supply.
What happened to money in Geanakoplos' model? How come there was inflation in a model with no money? Did I misunderstand something?
Edit: I know that we can talk about inflation in terms of relative prices against some numeraire good when there is no money. However, in the example provided by Geanakoplos in the video lecture, there was a single good, which is called "apple" by Geanakoplos. The confusing thing is, he defined inflation as the two-period ratio of the prices of an "apple", the only good in the economy! In such case, what is this ratio measuring? What is inflating exactly against what?