This article is a tribute to Phillip Anderson, who passed away on March 29, 2020. Besides his pioneering works in condensed-matter physics, he also wrote extensively on the theme of reductionism and multiscale physics.
Modern science reveals that matter is made of atoms and molecules. Molecules in liquids and gases move randomly; there is an average distance between two nearest molecules. This distance is used to model the properties of the gas. However, there are certain problems, like turbulence, that cannot be solved using just the distance, which is a single scale. We need to consider all scales from large to small. Such systems are called multiscale systems.