Next: High Energy Physics
From Democritos through the alchemists of the Middle Ages and Mendeleyev's Periodic Table of the elements all the way to modern elementary particle physics, one of the first duties of "natural philosophers" has been to make up lists of all possible constituents of matter - preferably (for the sake of simplicity) including only the irreducible components.
This notion may well be obsolete in the literal physical sense, but the concept lives on; and it is tempting (if misleading) to describe elementary particle physics as the art of inventing the simplest possible taxonomy (classification scheme) for the "zoo" of known "elementary" particles.
Objects or entities can only be classified in terms of their properties. Thus the first task is to define all the (known) intrinsic properties of matter as concisely as possible, invent ways of measuring how much of each property a given particle has, and do the experiments. Of course, this is an iterative process - after each round of experiments the theorists have to go back to their drawing boards and revise the Ultimate Classification Scheme - but the idea is still the same. My task is now to summarize in one handout over half a century of progress along these lines. Naturally I will omit as many of the false starts and backtracks as possible, to make it look as if the present scheme is correct and was obvious from the outset.