The idea that matter consists of smaller particles and that there exists a limited number of sorts of primary, smallest particles in nature has existed in natural philosophy at least since the 6th century BC. Such ideas gained physical credibility beginning in the 19th century, but the concept of "elementary particle" underwent some changes in its meaning : notably, modern physics no longer deems elementary particles indestructible. Even elementary particles can decay or collide destructively ; they can cease to exist and create (other) particles in result.