Nature includes all forms of matter and energy. Modern atomic theory is a scientific theory of the nature of matter, which states that matter is composed of discrete units called atoms. Since atoms (as defined by modern scientists) were found to be divisible, physicists later invented the term “elementary particle” to refer to particles that are truly indivisible (the original meaning of the ancient Greek word atom).¹

“If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied”. – Richard Feynman

Nature is governed by physical laws such as conservation laws, classical mechanics, and relativity. According to Einstein, the laws of conservation of energy and conservation of mass are “one and the same” — this principle is fundamental to many fields of physics, including nuclear and particle physics.²

There is a fundamental element of Chance active in Nature. It makes possible free will, moral responsibility and evolution. Without random small changes in the direction of atoms, we could chart the course of every atom in all of the universe to infinity exactly, leaving no possibility for choice or change.³

“Some believe that everything is ruled by fate, but we should dismiss this. One who is wise knows that the greater power of decision lies within oneself. He understands that while some things are indeed caused by fate, other things happen by chance or by choice. He sees that fate is irreproachable and chance unreliable, but choices deserve either praise or blame because what is decided by choice is not subject to any external power… [but he] does not believe that everything is randomly caused.” — Epicurus⁴

There is nothing “outside” of Nature. There are no supernatural gods or realms. There is no divine intervention not subject to the laws of nature. There is no spirit, or logos, or animating force that is not subject to the laws of nature.

There are no edges to Nature. Lucretius “invites us to assume that the universe is finite and to consider what would happen if someone went to the very edge of it and threw a spear. One of two things would happen: either the spear would be stopped, in which case there must be matter ahead; or the spear would fly on, in which case there must be space ahead. In neither case has the boundary of the universe been reached.”⁵

Next in the outline: There is no afterlife

1. ^α More on the modern atomic theory on Wikipedia. Note “The term elementary particle is now used to refer to particles that are actually indivisible.”
2. ^α More on mass-energy equivalence on Wikipedia.
3. ^α The Heisenberg Uncertainty Principle which “is inherent in the properties of all wave-like systems” (Wikipedia) is a fact of modern quantum physics which seems to point to the truth of this philosophical concept. Another is the related Quantum indeterminacy which is “a much more fundamental nature, having nothing to do with errors or disturbance.”
4. ^α From the Letter to Menoceus by Epicurus, preserved in Lives of Eminent Philosophers by Laertius Diogenes.
5. ^α From De Rerum Natura by Lucretius