Rediscovering what was once known

As I was re-reading The Effective Executive, a Drucker classic from 1966, I noticed something interesting.

He advises one should ask for opinions and then "test them against reality" by asking the following questions:

"What do we have to know to test the validity of this hypothesis?"

"What would the facts have to be to make this opinion tenable?"

"What is the criterion of relevance?"