Epicurus believed that there were twelve principles of nature, provable through firm evidence and true reasoning, using our five senses, our faculty of perceiving “anticipations,” and our “feelings” of pleasure and pain. He believed in conclusions supported by clear and convincing evidence. No evidence, he said, is ever to be disregarded as worthless. Real evidence is essential. Error occurs only in the mind, and where evidence about a matter is insufficient to give rise to a firm opinion, we must wait before labeling any opinion about the matter true or false.

Epicurus taught that we have no need to rely on any gods, priests, or supernatural claims for our understanding of Nature. What our faculties suggest to us is “true”. Only if we use them properly we can be confident in our conclusions. And we can only use our faculties properly if we understand the process by which they operate.

Epicurus concluded that the following twelve aspects of nature are crucial to understanding how both Nature, and our faculties, operate:

1. Matter is uncreatable.
2. Matter is indestructible.
3. The universe consists of solid bodies and void.
4. Solid bodies are either compounds or simple.
5. The multitude of atoms is infinite.
6. The void, or Space, is infinite in extent.
7. Atoms are always in motion.
8. The speed of atomic motion is uniform.
9. Motion is linear in space, vibratory in compounds.
10. Atoms are capable of swerving slightly at any point in space or time.
11. Atoms are characterized by three qualities: weight, shape and size.
12. The number of the different shapes is not infinite, merely innumerable.

Yes, today we know so much more, use a plethora of new names, techniques and instruments. And atoms are indeed divisible. But the reader can observe that the basics of the modern scientific method are there, and that, using “true reasoning” (the ancient Greeks did not even have a telescope) Epicurus was right that, at some fundamental level, the physical universe is composed of particles, and that any phenomenum that is observable to the senses exists as part of our own universe and is neither created by, nor is subject to, any supernatural forces.

The method by which these elemental observations were established can be found in Epicurus’ “Letter to Herodotus” and in Lucretius’ De Rerum Natura.

The above list of twelve elementals was reconstructed by Norman DeWitt in his book Epicurus and His Philosophy. A similar list has also been prepared by Professor Diskin Clay in his work Paradosis and Survival.
(Edited version of an article on New Epicurean.com, with thanks)