Economists know this because money stable in value needs to be able to expand and contract with its underlying economy.
There is no single group of economists with a single _opinion_. Economics is a social science, most of the predictions are not falsifiable. A lot of economists drive the agenda of their masters. A good example for successfully using economics as a weapon is Ken Rogoff. A very good example of a failing economic model can be currently seen in india.
timeo oeconomicus et dona ferentes