Be ready to revise any system, scrap any method, abandon any theory, if the success of the job requires it.
Experience without theory is blind, but theory without experience is mere intellectual play.
There is another theory which states that this has already happened.
No theory changes what it is a theory about; man remains what he has always been.
It has long been a theory of mine and I am known, if I do say so, for my long theories that authors, generally speaking, are rotten letter writers.
“In theory, there is no difference between theory and practice. But in practice, there is.”
In theory there is no difference between theory and practice. In practice there is.
He who loves practice without theory is like the sailor who boards ship without a rudder and compass and never knows where he may cast.
Read no history nothing but biography for that is life without theory.