“When Physicists speak of "beauty" in their theories, they really mean that their theory possesses at least two essential features: 1. A unifying symmetry 2. The ability to explain vast amounts of experimental data with the most economical mathematical expressions.”

Source: Reinventing Gravity (2008), Chapter 9, Other Alternative Gravity Theories, p. 143

A Treatise on Electricity and Magnetism (1873), §36.

After all control and institutions and processes are immediate things. They can all be translated into terms of human conduct...
Source: The Institutional Approach to Economic Theory, 1919, p. 311-6

"The Fundamentals of Theoretical Physics," (1940) as quoted in Out of My Later Years (1976)
1940s

The Evolution of the Physicist's Picture of Nature (1963)
Context: It seems to be one of the fundamental features of nature that fundamental physical laws are described in terms of a mathematical theory of great beauty and power, needing quite a high standard of mathematics for one to understand it. You may wonder: Why is nature constructed along these lines? One can only answer that our present knowledge seems to show that nature is so constructed. We simply have to accept it. One could perhaps describe the situation by saying that God is a mathematician of a very high order, and He used very advanced mathematics in constructing the universe. Our feeble attempts at mathematics enable us to understand a bit of the universe, and as we proceed to develop higher and higher mathematics we can hope to understand the universe better.

Thoughts on the Riemann hypothesis http://link.springer.com/article/10.1007/BF02985392 The Mathematical Intelligencer (December 2004) vol. 26, issue 1, pp. 4–7, quote on p. 4

The Elegant Universe (1999) Ch. 7 The "Super" in Superstrings.

Source: Just a Theory: Exploring the Nature of Science (2005), Chapter 2, “Just a Theory: What Scientists Do” (p. 24)

Source: Managers Not MBAs (2005), p. 362