“No great importance is to be given to mere experience.”

The Value of Science (1955)
Context: The scientist has a lot of experience with ignorance and doubt and uncertainty, and this experience is of very great importance, I think. When a scientist doesn’t know the answer to a problem, he is ignorant. When he has a hunch as to what the result is, he is uncertain. And when he is pretty darn sure of what the result is going to be, he is still in some doubt. We have found it of paramount importance that in order to progress we must recognize our ignorance and leave room for doubt. Scientific knowledge is a body of statements of varying degrees of certainty — some most unsure, some nearly sure, but none absolutely certain. Now, we scientists are used to this, and we take it for granted that it is perfectly consistent to be unsure, that it is possible to live and not know. But I don’t know whether everyone realizes this is true. Our freedom to doubt was born out of a struggle against authority in the early days of science. It was a very deep and strong struggle: permit us to question — to doubt — to not be sure. I think that it is important that we do not forget this struggle and thus perhaps lose what we have gained.

Preface.
A History of Science Vol.2 Hellenistic Science and Culture in the Last Three Centuries B.C. (1959)

Source: A Short History Of The English Law (First Edition) (1912), Chapter I, Old English Law, p. 11

This is declared to be "an old Kantian maxim" in General Systems Vol. 7-8 (1962)‎, p. 11, by the Society for the Advancement of General Systems Theory, but may simply be a paraphrase or summation of Kantian ideas.
Kant's treatment of the transcendental logic in the First Critique contains a portion, of which this quote may be an ambiguously worded paraphrase. Kant, claiming that both reason and the senses are essential to the formation of our understanding of the world, writes: "Without sensibility no object would be given to us, and without understanding none would be thought. Thoughts without content are empty, intuitions without concepts are blind (A51/B75)".
Disputed

Source: Love Bites

"The Nature of Freedom", item 94
Industrial Society and Its Future (1995)

Problems in Breast Pathology, W.B. Saunders Company Ltd London . Philadelphia - Toronto 1979, p. 113
Prof. Azzopardi was speaking about subtle problems in diagnosis of carcinoma of the breast, but his observation has a much larger significance.

Archimedes or the Future of Physics (1927)
Context: The question of the reversibility of natural processes provides the key to a great intellectual struggle which is now behind the complexities of philosophic and scientific thought. The issue can be formulated thus: Is there a real temporal process in nature? Is the passage of irreversible time a necessary element in any view of the structure of nature? Or, alternatively, is the subjective experience of time a mere illusion of the mind which cannot be given objective expression? These are not metaphysical questions that can still be neglected with impunity. For just as Einstein made his advance by analysing conceptions such as simultaneity, which had been thought to be adequately understood for the purposes of experimental science, so the next development of physical theory will probably be made by carrying on the analysis of time from the point at which Einstein left it.