ABSTRACT
Philosophers and scientists have long bothered themselves with a pair of related questions: What is the constitution of things? What kinds of things are there? The questions are related in this way. If there are some basic kinds of things, then the differences between less basic kinds of things can be explained in terms of the natures of the basic kinds. So, for instance, the ancients followed Empedocles in acknowledging four elements (earth, air, fire, and water) and attributed the existence and properties of different kinds of things to differing proportions of these elements. It was this picture that encouraged alchemists to think that they could transform base metals such as lead, mercury, and tin into more valuable ones such as silver and gold. For all its faults, their outlook has been vindicated in its most general features by modern physics and chemistry. Being chemical elements, these metals cannot be transformed into one another by chemical means. But they do have common constituents in terms of subatomic particles (electrons, neutrons, and protons) and the transformation of lead into gold has in fact been achieved by physical means.
