You can have both inflation (in terms of purchasing power) and a simultaneous decrease in the cost of goods (in constant dollar terms). If both of these happen

You can have both inflation (in terms of purchasing power) and a simultaneous decrease in the cost of goods (in constant dollar terms). If both of these happen to balance, prices appear to stay the same. Presumably you'd only get a true number out of something (a) with a fixed amount in circulation, (b) internationally traded, & (c) universally valued. Even precious metals are subject to production costs and industrial consumption.