The notion that there could be a scientific study of any phenomena rests upon two primary underlying assumptions: that a relationship of unity and diversity occurs within existence, and that this relationship can, at diverse times, be understood by the human interpretive element. The Greeks of Classical Athens began the first comprehensive attempt to understand the relationship between unity and diversity in a manner that we might recognize, at the very least, as proto-scientific. While this is most popularly understood as early philosophy and mathematics, many of what we would recognize as the core academic disciplines can be traced to the teachers of Classical Greece, and their animated pursuit of differing pathways of knowledge.
Among these varying disciplinary developments, History, as we would recognize it - being an arena of study which goes beyond chronicling, accounting, or mythology - is usually traced to the writings of Herodotus, the so called "Father of History." Though his recounting of the Greco-Persian War bears the seeds of what has become modern-day history, it would take another generation, and another war, for Thucydides to create what Charles Norris Cochrane calls a truly materialist history. While Herodotus often related the fantastic, amazing, and surely fanciful in his histories of the Greek Mediterranean, he who goes looking for those beasts and gods, divine causes and mythical actors in Thucydides will be truly disappointed. Hoping that his history of the Peloponnesian War could be a "possession for all time" Thucydides sought to relate the causes, and fighting of, the war as he understood it to be, so that he could contribute not only to the memory of it, but provide a service to the future. In attempting to develop a kind of political science within his history, Thucydides turned to experience as the only guide for us to understand what has been and what will be. Inasmuch as men and the world contain similarities across time and space, Thucydides' work serves as one of the earliest explorations of human action as the sole evidence for a better understanding of people as people. Not relying upon some first principle, or illusory other realm of explanation makes his work, as Cochrane argues, an attempt at developing a scientific approach to history.
While great historians like Gibbon and Herodotus (or lesser ones like Marx) relied upon principles of recursion, cycles, or recurrent dialectics, to explain historical patterns, Thucydides (and later historians such as Machiavelli) sought the development of theoretics solely in the world of the sensible. That this approach to history requires explicit differentiation may strike many as surprising. Certainly Toynbee ("history is just one damned thing after another") might contest that any other approach constitutes history proper, yet such philosophic or social scientific-inspired history plays a not inconsequential role in the contemporary academic field. This is not to argue that Thucydides, Toynbee, or Machiavelli are value-neutral, far from it. Rather, an explicit acquaintance with the presuppositions which color our histories are as inescapable as those which color all other sciences. While we may not be able to predict and replicate in history as, say, a physicist or mathematician can, post-Darwinian science has broadened our horizon of the sciences. Rather than prediction, explanation becomes more the guiding principle of scientific investigation. Here we are on Thucydides' home-ground, and commonalities across arenas of knowledge can be fruitfully pursued. As conceptions of the sciences broaden, enfolding numerous scales of human experience and expression, the lessons of history and the sciences crosspollinate, to form, in concert, a more complete accounting of the world and the human element within it.