“If to-day you ask a physicist what he has finally made out the æther or the electron to be, the answer will not be a description in terms of billiard balls or fly-wheels or anything concrete; he will point instead to a number of symbols and a set of mathematical equations which they satisfy. What do the symbols stand for? The mysterious reply is given that physics is indifferent to that; it has no means of probing beneath the symbolism. To understand the phenomena of the physical world it is necessary to know the equations which the symbols obey but not the nature of that which is being symbolised. … this newer outlook has modified the challenge from the material to the spiritual world.”

Source: Computer Science as Empirical Inquiry: Symbols and Search (1975), p. 116. This is also called the Church–Turing thesis.

Science and the Unseen World (1929), VII, p.73

Science and the Unseen World (1929)

The external world of physics has thus become a world of shadows. In removing our illusions we have removed the substance, for indeed we have seen that substance is one of the greatest of our illusions. Later perhaps we may inquire whether in our zeal to cut out all that is unreal we may not have used the knife too ruthlessly. Perhaps, indeed, reality is a child which cannot survive without its nurse illusion. But if so, that is of little concern to the scientist, who has good and sufficient reasons for pursuing his investigations in the world of shadows and is content to leave to the philosopher the determination of its exact status in regard to reality. In the world of physics we watch a shadowgraph performance of the drama of familiar life. The shadow of my elbow rests on the shadow table as the shadow ink flows over the shadow paper. It is all symbolic, and as a symbol the physicist leaves it. Then comes the alchemist Mind who transmutes the symbols. The sparsely spread nuclei of electric force become a tangible solid; their restless agitation becomes the warmth of summer; the octave of aethereal vibrations becomes a gorgeous rainbow. Nor does the alchemy stop here. In the transmuted world new significances arise which are scarcely to be traced in the world of symbols; so that it becomes a world of beauty and purpose — and, alas, suffering and evil.
The frank realisation that physical science is concerned with a world of shadows is one of the most significant of recent advances.

Introduction
The Nature of the Physical World (1928)

The Mathematical Way of Thinking (1941)

Kosmos (1932)

Source: The Fabric of Reality (1997), Ch.3

Science and the Unseen World (1929)
Context: It remains a real world if there is a background to the symbols—an unknown quantity which the mathematical symbol x stands for. We think we are not wholly cut off from this background. It is to this background that our own personality and consciousness belong, and those spiritual aspects of our nature not to be described by any symbolism... to which mathematical physics has hitherto restricted itself.<!--III, p.37-38

Source: Morals and Dogma of the Ancient and Accepted Scottish Rite of Freemasonry (1871), Ch. III : The Master, p. 63
Context: To present a visible symbol to the eye of another, is not necessarily to inform him of the meaning which that symbol has to you. Hence the philosopher soon superadded to the symbols explanations addressed to the ear, susceptible of more precision, but less effective and impressive than the painted or sculptured forms which he endeavored to explain. Out of these explanations grew by degrees a variety of narrations, whose true object and meaning were gradually forgotten, or lost in contradictions and incongruities. And when these were abandoned, and Philosophy resorted to definitions and formulas, its language was but a more complicated symbolism, attempting in the dark to grapple with and picture ideas impossible to be expressed. For as with the visible symbol, so with the word: to utter it to you does not inform you of the exact meaning which it has to me; and thus religion and philosophy became to a great extent disputes as to the meaning of words. The most abstract expression for Deity, which language can supply, is but a sign or symbol for an object beyond our comprehension, and not more truthful and adequate than the images of Osiris and Vishnu, or their names, except as being less sensuous and explicit We avoid sensuousness, only by resorting to simple negation. We come at last to define spirit by saying that it is not matter. Spirit is — spirit.