“The one intelligible theory of the universe is that of objective idealism, that matter is effete mind, inveterate habits becoming physical laws. But before this can be accepted it must show itself capable of explaining the tridimensionality of space, the laws of motion, and the general characteristics of the universe, with mathematical clearness and precision; for no less should be demanded of every Philosophy.”

Reported in: Memorabilia Mathematica by Robert Edouard Moritz, quote #129.

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

Source: Mathematics and the Physical World (1959), pp. 224-225

Literary and Historical Miscellanies (1855), The Necessity, the Reality, and the Promise of the Progress of the Human Race (1854)
Context: The unchanging character of law is the only basis on which continuous action can rest. Without it man would be but as the traveller over endless morasses; the builder on quicksands; the mariner without compass or rudder, driven successively whithersoever changing winds may blow. The universe is the reflex and image of its Creator. "The true work of art," says Michael Angelo, "is but a shadow of the Divine perfections." We may say in a more general manner, that Beauty Itself Is But The Sensible Image Of The Infinite; that all creation is a manifestation of the Almighty; not the result of caprice, but the glorious display of his perfection; and as the universe thus produced, is always in the course of change, so its regulating mind is a living Providence, perpetually exerting itself anew. If his designs could be thwarted, we should lose the great evidence of his unity, as well as the anchor of our own hope.
Harmony is the characteristic of the intellectual system of the universe; and immutable laws of moral existence must pervade all time and all space, all ages and all worlds.

Space, Time and Gravitation (1920)

Editor's Introduction, The Teaching of Elementary Mathematics https://books.google.com/books?id=NKoAAAAAMAAJ (1906) by David Eugene Smith

David Eugene Smith, "Editor's Introduction," in: The Teaching of Elementary Mathematics https://books.google.com/books?id=NKoAAAAAMAAJ (1906)

Source: The Other Side Of The Coin (2008), Chapter 3, One Versus Plurality, p. 89

Notice sur les Titres et Travaux scientifiques de Pierre Duhem rédigée par lui-même lors de sa candidature à l'Académie des sciences (mai 1913), The Aim and Structure of Physical Theory (1906)