Alfred North Whitehead famously said that all of Western philosophy was nothing more than a series of footnotes to Plato. This statement is a bit deceiving. There were many creative Western philosophers after Plato, bringing unique ideas into the history of philosophy. The general idea that he was trying to express was that Plato was the first lengthy writer, one who was very influential on another great philosopher who was his student, namely Aristotle. Every future philosopher in the West took up some position that was somewhat positive or negative regarding these figures.

Thus, even many centuries later, the early modern rejection of Medieval philosophy was actually also a rejection of Aristotle in many ways, for Aristotle was a key component of medieval learning. Interestingly, at this same time, there was a great uptick in studies of Plato among humanist scholars. Still, even in utter rejection of some previous thinker, one must have considered that thinker at some point and at some depth. This keeps some "root" in the past even while rejecting it. Thus, even thousands of years later, philosophers write "footnotes of footnotes of footnotes" concerning the thought of Plato—at least in a broad sense of speaking!

In his The Course of Positive Philosophy, Auguste Comte famously divided the history of societies up into three main periods. These periods are said to be characterized by their manners of explaining the causes of human experience. The first is the theological phase. In this stage, causes are explained primarily in terms of gods or a god. Then, in the "metaphysical" stage, abstract notions are used to explain the causes of things. Finally, in the "positive" stage, only "positive" (or posited) facts and scientific observations are the primary mode of explanation. Comte thought the final type of society was the highest and most developed. By doing so, he set up his own time's philosophical conception of knowledge as the pinnacle of human thought.

In a way, all of these thinkers were influential, though in different ways. Francisco de Vitoria was a teacher in Spain whose work on natural rights is part of a broader discussion that would eventually filter through many Catholic and Protestant thinkers. These thinkers would become sources for the pivotally important Thomas Hobbes, whose best known political work is the Leviathan—a brutal but fully developed treatise on a quite domineering notion of the nation state. Likewise, Baron de Montesquieu was quite influential on many political thinkers during this period, as was Jean-Jacques Rousseau.

The most important thinker concerning the Declaration of Independence is John Locke. It is from Locke's thought that Thomas Jefferson derived his remarks regarding the rights to "life, liberty, and the pursuit of happiness." In Locke's political philosophy, he actually presents life, liberty, and property as the three fundamental rights of human persons. Locke's position was a kind of softening of the much harsher position of Hobbes, who stated that when we are not in society, we only have one fundamental right—self defense! Note, of course, that Jefferson changed "property" to "pursuit of happiness." This followed the recommendations of his fellow drafters, who hoped thereby to avoid issues that could have arisen because of a very problematic form of property in the colonies—slaves.

In his political philosophy, Jean-Jacques Rousseau discusses (among other things) the idea of the "general will" of the people. This idea can seem very horrifying—almost like a tyrannical will of the majority over the rest of the people. He means something much simpler than that. He is merely referring to the idea that in a given group of people, the people as a whole have a will that is aimed at certain common goals. Indeed, in some ways, Rousseau's thought in this regard is a kind of strange version of what some Aristotelians and Medieval Scholastics meant in their discussions of how the common good is willed by a political body. Of course, there are great differences in how they conceived of the common good and how Rousseau discussed the "general will." What remains, however, is the fact that Rousseau is, in fact, well known for using this notion in his political philosophy.

The probably apocryphal Lao Tze was the author of the book of poetry the Tao Te Ching, which is the foundational text for the philosophy known as Taoism. While many passages are ambiguous and worded in an unusual way, the philosophy largely argues for allowing the "Tao," a spiritual force, to flow through a human's life. The book heavily influenced later Chinese thought, such as Confucianism and Chan Buddhism.

The Poetics were the first comprehensive approach to defining the key elements and requirements of the dramatic form. Tellingly, its author, Aristotle, was a great philosopher, rather than a great playwright or poet. The work features Aristotle's deep, systematic thinking, applied specifically to what is widely considered the "poetic arts," including lyric poetry, epic poetry, tragedy, comedy, and the satyr play.

The great Aristotle was the student of an equally great philosopher, namely Plato. Indeed, Aristotle spent 20 years at Plato's Academy! He didn't do this merely because he was a slouch who didn't want to get a job. Instead, during this many years of learning, Aristotle imbibed much of Plato's thought. His later writings would in many ways temper and limit the excesses of his master. Nevertheless, when one reads Aristotle, one can sense the discussions of the Academy as being directly behind the scenes of Aristotle's own concerns.

Augustine of Hippo (354-430) has probably had more influence on Western society than almost any other person in all of history. This is almost certain when we limit ourselves to authors of Latin texts. After many years as a successful rhetorician (living a dissipate lifestyle), Augustine converted to Christianity. He was soon the Bishop of Hippo and became a prolific writer. His thought would influence Western Christianity throughout all of the Middle Ages and would be an important touch point throughout numerous reformations and revolutions within Western Christendom down to the present day. Indeed, during the Middle Ages, for example, there were two great authorities—the Greek man, Aristotle, and Augustine.

In the Confessions, Augustine tells the tale of his conversion—giving a long history of his life as well as his beliefs. This text is amazing for its personal depth and is a unique testimony of someone's psyche during the period of late antiquity.

Interestingly, one of the great founders of philosophy did not actually write anything that we know of. Almost all that we know of him comes from the reports we have received from Plato as well as the play Clouds by Aristophanes. The bulk of the material, in any case, comes from Plato. In his dialogues, Plato portrays Socrates as discussing philosophical matters with various Athenians. Likely, Plato puts some of his own positions on the lips of Socrates. Nevertheless, if we want to puzzle out just what was the "philosophy of Socrates" our only real source can be found in Plato's dialogues.

The Pre-Socratics are so named merely because of their place in history. They came before Socrates. Socrates is like a "dividing line" for Greek philosophical history! In any case, the Pre-Socratics like Thales and Anaxagoras were interested in the basic elements from which all things were made and how those elements became what we know of in our experience. Other thinkers like Heraclitus and Parmenides were interested in more esoteric questions about being and non-being. In some ways, these thinkers were interested in "metaphysics"—especially Parmenides and Heraclitus. More broadly, the group was interested in cosmology or the way the world is to be understood philosophically. Hence, the best overall category for them is that of "natural philosophy", understanding that all of these terms were coined after the time of the pre-Socratics. With Socrates, philosophy took on a very human-centered and ethical set of concerns.