And you know who else agrees with Mr. Keynes on this definition, VON MISES, quoted directly from Theory of Money and Credit, at
http://archive.mises.org/19306/inflation-and-deflation-austrian-definitions/an increase in the quantity of money (in the broader sense of the term, so as to include fiduciary media as well), that is not offset by a corresponding increase in the need for money (again in the broader sense of the term), so that a fall in the objective exchange-value of money must occur.
That's a quote taken well out of context. Mises would acknowledge the semantics of price inflation/deflation, but he would stick to the supply issue of it.
But as you said, that's all semantics.
I'm responding to you to mention that there is in fact reason to look at the supply of money, and more importantly, changes in that supply. This is because money is a non-neutral good, and prices take time to change through an economy.
If you are to expand the supply of money exactly with the population, it isn't as simple as just saying that prices will adjust accordingly. You must know the qualitative issues that surround the means by which the supply of money was increased. If that money goes first to X individual, then that individual benefits most from the newly created money regardless of the aggregate data surrounding the change involved. Likewise, person Y who receives the new money (as in, who is last to adjust to the change in supply,) is actually made worse off because of the lower purchasing power of their money in the time until prices adjusted.
This is why the issue of "price stability" is really impossible to answer for the "best" outcome. There is no such thing as price stability when the market of millions of goods have independently changing prices. You may be able to make aggregates of the prices (CPI, PPI, etc,) but those aggregates hide the meaningful information that prices give to an economy.