Exactly what is substance? Until comparatively recently this question has been considered to be synonymous with "What is matter?" and it has attracted the attention of scientific thinkers for at least twenty-five centuries.
At the early beginnings of Greek philosophy matter was a mystery. Then, by imagining what would happen if a piece of it were divided into smaller and smaller particles, the philosophers arrived at a concept of the minutest morsel possible, something that could not be further divided, and called it an "atom." Later philosophers developed this concept into an atomic theory of matter, which was to rouse the imagination of the Renaissance and be picked up, in a general way, by the new scientists of the early seventeenth century. By the first quarter of our present century, a more detailed picture had emerged of the atom as a tiny "solar system" consisting of a nucleus with electrons spinning around it.
Another few years, and the theoretical explanation for this view, called quantum electrodynamics, seemed to have almost completed the definition of matter. But mysteries still remained, and as these were probed more deeply, a profusion of other strange particles appeared—only to reveal in their turn more mysteries for the physicists to pursue at an even deeper level. Recently the theory that the basic particles in the nucleus of the atom are made up of even smaller entities called quarks, the newest "indivisible" particle, was propounded, and evidence began to accumulate to support their existence. But forward-looking physicists were already asking themselves: If these subatomic particles exist, can it be assumed that they will turn out to be the basic building blocks of matter? Will deeper research reveal substances even more elementary?
