It seems easy to deal in a general way with civilizations as entities; at least this is commonly done. Arnold Toynbee, in his _Study of History_, distinguishes without hesitation twenty-one civilizations—“specimens of the species,” belonging to the “genus societies”—by what he believes to be an empirical method. But consider the problem which arises when we want to study the genesis of any one civilization in particular! We cannot merely assume that it is an entity and has a recognizable character of its own; we are bound to make that character explicit in order that we may decide _when_ and _where_ it emerged. This problem is hardly ever envisaged by those who are best acquainted with the actual remains of antiquity. The archaeologist is either occupied with disentangling successive phases in his stratified material; or he constructs from his finds a fairly continuous story of man’s increasing skill and enterprise. In this context the questions _when_ and _why_ we are entitled to speak of the existence of Egyptian or Sumerian (_i.e._ early Mesopotamian) civilization seem of secondary importance. On the other hand, the philologist does not encounter the question at all. For him, Sumerian or Egyptian civilization exists from the moment when texts were written in these languages. Our problem is pre-eminently a historical one, and it has, accordingly, two aspects: that of _identity_ and that of _change_. What constitutes the individuality of a civilization, its recognizable character, its identity which is maintained throughout the successive stages of its existence? What, on the other hand, are the changes differentiating one stage from the next? We are not, of course, looking for a formula; the character of a civilization is far too elusive to be reduced to a catchword. We recognize it in a certain coherence among its various manifestations, a certain consistency in its orientation, a certain cultural “style” which shapes its political and its judicial institutions, its art as well as its literature, its religion as well as its morals. I propose to call this elusive identity of a civilization its “form.” It is this “form” which is never destroyed although it changes in the course of time. And it changes partly as a result of inherent factors—development—partly as a result of external forces—historical incidents. I propose to call the total of these changes the “dynamics” of a civilization. The interplay of form and dynamics constitutes the history of a civilization and raises the question—which lies outside our present inquiry—to what extent the form of a civilization may determine its destiny.