The Machian approach requires physics to be constructed from directly observable quantities.

Many researchers were working on the theory of observable quantities in the 1940s.

From the experimental viewpoint the observable quantity is the cross section.

One is theoretically free to make the assignment in either way, and no observable quantity can depend on this choice.

In particular, all physically observable quantities must be independent of such choices.

Perhaps this observation begins to afford a clue to how it comes about that observable quantities can be associated with certain operators.

My paper proposed a theory that in principle enabled one to calculate the values of certain observable quantities.

The actual number of accidents experienced by a customer is the observable quantity.

An operator S has been associated with an observable quantity, the angular momentum.

However this is not an observable quantity, since the future return on any asset can never be known with complete certainty.