--Abraham Pais, 'Subtle is the Lord...': The Science and the Life of Albert Einstein (1982). The full names of the men mentioned by Pais are Max Abraham, Hendrik A. Lorentz, and Henri Poincare. See thesis for a discussion of their contributions.
"I do not believe that the problem of matter is to be solved by a mere field theory."
--Hermann Weyl, Gravitation and Electricity (1923). Weyl was a mathematical physicist (unlike Einstein, who had help developing the math involved in general relativity) who became an expert in general relativity.
"...'matter' has lost its role as a fundamental concept."
--Albert Einstein, Relativity: The Special and the General Theory. This is a book Einstein wrote for the general public in the early 1920s. The comment here comes from a final short chapter added to the last edition of the book, published in 1952, three years before Einstein died.
"Physics is the study of the fundamental laws of nature, but what constitutes a law and which laws are taken to be fundamental are matters of evolving consensus among physicists."
--Richard A. Matzner and Lawrence C. Shepley, Classical Mechanics (1991). Matzner and Shepley are physicists at the University of Texas at Austin.