The lack of a good money being available to use as unit of account (UoA) makes reasoning about interest rates a bit cumbersome, so I’ll try to clarify some concepts using a naive version of the formulas that explain their relationship.
An ideal UoA asset would be one characterized by a constant relative demand in the economy, no cost of storage and no friction to be exchanged for other goods and services. Such monetary asset will increase in purchasing power at the same rate as the production of goods and services grows.
price_inflation = -productivity_growth
Human beings will always try to accomplish their goals as soon as possible, and they will only be willing to postpone the attainment of their ends when they subjectively believe that by doing so they will achieve more valuable objectives 1. Hence, even in the absence of risk, any loan will have a time preference premium. In purchasing power terms we can say that:
real_ir = productivity_growth + time_preference + risk_premium