The interest rate is the continuous "own rate" of inflation

In chapter 17 of keynes' general theory, various ideas of interest are discussed. In particular, he uses a concept of the "own rate". This is the interest of any good or service, denominated in that same unit. So the own rate for apples, is how many apples you have to sell today to lock in a futures contract for some fixed amount of apples at a future date.

Like many rates, this "own rate" relates to other rates, such as interest or inflation, in specific ways. While we can get into the mathematics of all that, for now, let's consider the different ways that inflation is measured.

Inflation can be measured as a change in the available stock of a commodity money. It can be measured through exchange rates to other currencies, or it can be measured as a change in purchasing power for a fixed basket of items, whether those are commodities or anything else.

Another way to consider inflation, is merely "how does the purchasing power of our money unit change over time". For this, we can merely measure "how many dollars can I buy in a year, for a dollar today".

Under this viewpoint, the interest rate is not merely related to inflation, it is a specific **metric** for inflation. Like various other inflation metrics such as CPI or a basket of foreign currencies, these different inflation measurements should tend to correspond.

The price of money is the valuation of collateral, not a self referential temporal exchange in the same unit of account. That exchange is rather "the continuous instantaneous own rate of inflation".