The history of the physics of the small is in great measure the story of the discovery of new levels of structure, each associated with the development of a new set of experimental tools, and ultimately with a new discipline. We have repeatedly categorized some set of particles as the elementary constituents of matter, only to discover later that these should properly be considered as composed of yet‐more‐fundamental particles. Discounting the prescientific view of matter as composed of Air, Fire, Earth, and Water, five levels of structure have been discovered so far, as shown in table 1. Most readers of this article are probably familiar with the evidence for each level of structure; a good review of this evidence can be found in reference 1.

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