The beginnings of what we now call philosophy and natural science in the Western world are associated with a Pre-Socratic philosopher named Thales.
Thales is credited with developing the first "philosophical" or "scientific" theory. His theory is best understood as an account or explanation -- one which seeks to describe and explain how the world has come to be as it is.
Thales' account sounds quaint, if not naive to our ears:
All things are of water
While this sounds simple-minded, consider the sense it makes. It is not hard to think of all things in the world as divided into three classes -- solids, liquids, and gases. And water is known from direct experience to exist in each of these states. Perhaps the "underlying unity" of all solids, liquids, and gases is -- water, precisely the element known to transform into each state?
And notice what makes this theory "scientific" or "philosophical." Unlike religious "explanation" which appeals ultimately to the mysterious gods/goddesses/God -- Thales' account appeals to what may be known by way of observation and argument.
For more on this point, see the associated materials on explanation;
this Web also includes a more complete sketch of Thales' life and thought.