There are tens of thousands of chemical compounds of varying degrees of complexity. Understanding this huge number of systems would be a hopeless task if it were not for the underlying simplicity of the 116 fundamental units of which these compounds are made and the relatively small number of types of bounds through which they can interact. To understand chemistry, we need not study the properties of tens of thousands of compounds, but only those of about 100 elements, along with a few basic types of bonds between them.

In fact, the task is even simpler. The 116 known elements can be classified into groups with similar properties inert gases, halogens, alkali metals, transition metals, re earths, and so forth. If we understand the properties of one member of a group, we can infer the properties of the other members of that group.

The subatomic world can be understood in a similar way. We know that the 116 different kinds of atoms are not fundamental units, but rather they are in turn composed of three different particles protons, neutrons, and electrons.

When we look still further, by smashing particles together at high energy and studying the debris of the collisions we find what appears at first glance to be a complexity approaching that of chemistry hundreds of different particles are produced. Yet when we look carefully we find that we can classify those particles into a few groups whose members have similar properties. Eventually we find that this classification leads to clue about the underlying substructure that is based again on a small number of truly fundamental particles and small number of possible interactions among them.