I currently deal with interest rate theories, reading amongst others Paul Samuelson (1958) and Peter Diamond (1965).
In Samuelson the possibility of a naturally occuring negative interest rate is introduced due to consumption smoothing and without some central bank mechanism. Often times I heard (or read) that the interest rate is impossible to decrease below zero as soon as some commodity is introduced that preserves its value over time, like land or some kind of neutral money. What I don't understand is, why? What is the idea behind that and what kind of concept is this neutral money as opposed to "traditional" money?