An expert is a man who has made all the mistakes which can be made, in a narrow field.

An expert is someone who knows some of the worst mistakes, which can be made, in a very narrow field.

An expert is a person who has made all the mistakes that can be made but in a very narrow field.

Everything we call real is made of things that cannot be regarded as real.

Every description of natural processes must be based on ideas which have been introduced and defined by the classical theory.

While the finish given to our picture of the world by the theory of relativity has already been absorbed into the general scientific consciousness, this has scarcely occurred to the same extent with those aspects of the general problem of knowledge which have been elucidated by the quantum theory.