Atomism is a natural philosophy that proposes that the physical universe is composed of fundamental indivisible components known as atoms. The word atom comes from the Greek word ἄτομον (atomon), meaning “uncuttable” or “indivisible”. Atomism has its origins in ancient Greek and Indian philosophical traditions, and was later developed by various thinkers in the history of science and philosophy.

According to atomism, atoms are too small to be perceived by human senses, and they differ only in shape, size, and motion. They combine with each other by juxtaposition to form various observable forms and phenomena in nature. Atomism is an analytical doctrine that explains the properties of the whole by those of its parts and their configurations. Atomism is also associated with a realistic and mechanistic view of the world, which holds that atoms exist in actual reality and that all observable changes are caused by motions of the atoms.

Some of the main proponents of atomism in antiquity were Leucippus and Democritus, who are considered to be the founders of atomism in the 5th century BC. They argued that it is impossible to divide matter infinitely, and that matter must therefore be made up of tiny particles that cannot be further subdivided. They also proposed that there are two fundamental principles in nature: atom and void, or empty space. The variety of things in the world results from different shapes, arrangements, and positions of atoms in the void.

Another influential atomist was Epicurus, who adopted and modified the atomism of Democritus in the 3rd century BC. He added the idea that atoms can swerve randomly from their straight paths, which he used to explain free will and human agency. He also developed a hedonistic ethics based on atomism, which taught that the goal of life is to attain pleasure and avoid pain.

Atomism was also developed in ancient India by various schools of thought, such as the Vaisheshika school, which posited six categories of reality: substance, quality, action, generality, particularity, and inherence. Among these substances were atoms (anu), which were considered to be eternal, indivisible, and qualitatively identical. They combined with each other to form complex substances (dharma), which were subject to change and dissolution.

Another school of Indian atomism was Buddhism, which proposed a theory of momentary atoms (kalapa) that flash in and out of existence. These atoms were not considered to be material entities, but rather bundles of qualities or properties that constitute the basic units of experience. They were also subject to impermanence (anicca), suffering (dukkha), and non-self (anatta).

Atomism was revived and transformed in the early modern period by various natural philosophers and scientists, such as Pierre Gassendi, Robert Boyle, Isaac Newton, John Dalton, and others. They used atomism as a way to explain chemical reactions, physical laws, and empirical phenomena. They also introduced new concepts and methods, such as experimental evidence, atomic weights, molecular structures, and chemical symbols.

However, modern atomic theory differs from ancient atomism in several ways. For instance, modern atoms are not indivisible or qualitatively identical; they can be split into subatomic particles (such as protons, neutrons, and electrons) and they have different chemical properties depending on their atomic number. Modern atoms are also not mechanistic or deterministic; they exhibit quantum phenomena (such as wave-particle duality, uncertainty principle, and superposition) that defy classical physics.

Nevertheless, atomism remains a powerful and influential idea in philosophy and science. It has inspired many fields of inquiry and discovery, such as chemistry, physics, biology, cosmology, logic, ethics, psychology, sociology, and more. It has also raised many philosophical questions and challenges about the nature of reality, knowledge, causation, free will,
identity,
and morality.