No matter how complex modern physics concepts get, they have their roots in basic classical concepts. To understand the revolutions leading up to string theory, you need to first understand these basic concepts. You’ll then be able to understand how string theory recovers and generalizes them.
The question of matter’s meaning dates back to at least the Greeks and Chinese philosophers, who wondered what made one thing different from another. Greek and Chinese thinkers noticed similar trends, and each devised a system for categorizing matter into five fundamental elements based on these common traits.
In ancient China, the five elements were metal, wood, water, fire, and earth. Eastern religion and philosophy used these elements and the different ways they interact to explain not only the natural world but also the moral realm.
Among the Greek philosophers, Aristotle is the most popular to have discussed their version of the five elements: fire, earth, air, water, and aether. Aether was supposedly an unearthly, spiritual substance that filled the universe. In this view of matter, the realm outside of Earth was composed of this aether and didn’t undergo change the way our world does.
On Earth, material objects were seen as combinations of the basic elements. For example, mud was a combination of water and earth. A cloud was a combination of air and water. Lava was a combination of earth and fire.
In the 17th century, scientists’ understanding of matter started to change as astronomers and physicists began to realize that the same laws govern matter both on Earth and in space. The universe isn’t composed of eternal, unchanging, unearthly aether, but of hard balls of ordinary matter.
Newton’s key insight into the study of matter was that it resisted change in motion. The degree to which an object resists this change in motion is its mass.