“In many cases, the theory in question is too difficult to apply, and so scientists tend to base their work on models and approximations. The full acceptance of this fact has produced a subdiscipline that studies the nature of scientific models.16 And yet, as I argue in this book, the periodic table of chemistry is neither a theory nor a model but more akin to an “organizing principle,” for want of a better term. This book is partly an attempt to encourage philosophers of science to study the periodic table as an example of yet another scientific entity that does a lot of useful scientific work without being a theory.17”