ABSTRACT
We want to describe how we stand today in the age-old attempt to explain all of nature in terms of the working of a few elements in infinite variety of combinations; in particular, what are the elements? The many unexpected results of high-energy experiments have revealed the incompleteness of our knowledge of these elements. We shall describe those theoretical ideas which have been most useful in partially sorting out this material. We shall concentrate on the ideas themselves and shall not have time to discuss carefully their origin or the history of their development. Furthermore, we can only describe how things seem at present. Every sentence might be prefaced by: Of course it might turn out to look quite different, but …. We are well aware of the fragility and incompleteness of our present knowledge and of the manifold of speculative possibilities, but the exposition becomes awkward if we must continually refer to them. This is a survey of work from all over the world and not a report on any new contribution by the authors.
