We may describe the theist/atheist dichotomy in terms of dualism and monism.
To the theist, all existence divides into two domains: the natural and the supernatural, with the supernatural superior and occasionally intervening and altering the natural course of events. Thus, the natural world proceeds according to its own internal laws unless the supernatural miraculously intervenes to raise someone from the dead, or divide a sea's waters, or stop the sun in the sky. Thus, theists have a dualistic (i.e., natural/supernatural) view of the world.
Ancient philosophy once held a dualistic view of existence. Persuaded by Aristotle, philosophers divided the universe into two domains: the terrestrial domain and the celestial domain. Four elements—earth (soil), water, air, and fire—composed material things on Earth, i.e., in the terrestrial domain. A fifth element, the aether, composed the moon, planets and stars, embedded in concentric spheres in the celestial domain. Aristotle's teachings embody a type of dualism where earthy things and celestial things each have their distinct substances: earth, water, air, and fire for the earthly domain; aether for the celestial domain. Because different substances composed the two domains, ancient philosophers had no reason to expect what they learned about mundane things would apply to the heavens, too.
Some two millennia later, Newton said the same force that pulls an apple to Earth also keeps the moon and planets in orbit. Newton's theory of gravitation implicitly denies the dualism of Aristotle. Instead, it assumes the physical laws we see on Earth rule the heavens as well.
Newton's theory expresses one of science's bedrock principles, the uniformity of nature. To illustrate, suppose we observe the spectroscopic signature of neon in the light of a star a billion light-years distant. We conclude the star contains the element neon (or, more precisely, contained neon a billion years ago when it emitted the light). We know that on Earth, today, neon has that spectroscopic signature. But the star lies a billion light-years from Earth and the light we observe left the star a billion years ago. At that distant time and place, might not nitrogen or carbon have emitted light with the signature we observe? What ensures that the signature neon had a billion years ago, in a part of the universe a billion light-year distant, matches the one it has today? The principle of the uniformity of nature.
Uniformity of nature suggests, but does not prove, the philosophical position of monism, the view that a single entity or substance ultimately comprises all material entities. In other words, monism logically implies the uniformity of nature, but the uniformity of nature doesn't necessarily imply monism. (For instance, if the chemical elements were irreducible, the uniformity of nature might still obtain.)
Another line of thought suggests monism.
Consider what science tells us about material objects. Let's call the number of physical objects on Earth N1. Now imagine the number of distinct chemical compounds that comprise all those physical objects; we call it N2 (about ten million, by one estimate). We know N2 is less than N1, because, for instance, N1 includes millions of individual grains of salt but N2 has just one entry, sodium chloride. Now imagine the number of elements which comprise all the different chemical compounds; we'll call it N3. As of this writing, N3 equals a hundred and eighteen. Next, imagine all the subatomic particles that comprise all the chemical elements; we call it N4. As of this writing, N4 equals seventeen.
Given the trajectory of N1 to N2 to N3 to N4—of huge, to less huge, to a hundred and eighteen, to seventeen—we might imagine an endpoint of one: we might imagine that a single physical entity ultimately comprises all physical objects. In fact, some people view energy as the physical entity that comprises all physical objects (while others view it as a useful theoretical construct but refuse to call it the ultimate basis of all the physical universe).
Although science does not explicitly affirm monism, a monist view of the universe apparently accords with science. For instance, one description of the big bang says initially only one thing existed, energy, but as the universe expanded and cooled, energy condensed into protons, neutrons, electrons, and, eventually, us.
Our fundamental assumption: monism
In later chapters we assume a monist view of the universe. We'll regard each and every natural entity as a manifestation of a single, fundamental entity. Our assumption has many consequences, as we'll see.
But does monism bring us any closer to a definition of God, and if so, how? We need to explore a few philosophical ideas before answering that question.