Although he is also known for his work Utilitarianism, Mill's On Liberty has also been quite influential. Among other things in the work, Mill discusses the just limits that might be placed on an individual by the state. He contends that it is only just for the state to limit human freedom insofar as an individual is a danger to others. This outlook is held by many people today, and while Mill is not the only person from whom it derives, his work remains an important source for any such doctrine that would defend such a high amount of individual liberty in public life as a positive social good.

Although Aquinas and Augustine surely reflected on the biblical figure Abraham, it was Søren Kierkegaard who is most famous for his work Fear and Trembling. In that work, he undertakes a series of dramatic reflections on the role of anxiety, faith, and solitude in the life of Abraham, who was asked to by God to kill his only son (indeed, the son of the promises made to Abraham by God). The work reflects on how the individual person is placed "face-to-face" with God and with his or her moral destiny in the course of human life.

Suetonius is most famous for his history of the Roman Empire, De Vita Caesarum, widely known in English as The Twelve Caesars. Marcus Aurelius was an Emperor himself, also known for his philosophical work, The Meditations. Juvenal and Plautus were Roman comedy authors, while Aeschylus was a Greek playwright.

Plato's works were largely written as dialogues, conversations between two different thinkers on a weighty topic. In most of the dialogues, the key figure was Plato's teacher, Socrates. Socrates is considered one of the leading progenitors of Western philosophy, largely thanks to the writings of Plato.

While all the Roman Emperors read philosophy and a few others wrote literary works, only Marcus Aurelius, emperor from 161-180 CE, actually wrote a philosophical tract, known as The Meditations. Marcus Aurelius' Meditations were a development on Stoic philosophy, and the work is really a collection of thoughts and quotations compiled while Marcus Aurelius was Emperor and leading the Roman Army on campaigns.

De Oratore was considered a model of how to conduct rhetoric and oratory, not only in the ancient world, but throughout European history into the Renaissance. Its author, Cicero, became widely beloved in the Middle Ages and beyond for his philosophical focus on the engaged moral and noble man.

The three great Greek philosophers are uniquely tied together, through a series of teacher-student relationships. Socrates, the great founder of Greek philosophy, directly taught Plato, who recorded most of Socrates' thought. Plato then taught Aristotle himself, which makes the entire chain of Greek philosophy tied to all three.

Located in his work The Republic, the allegory of the cave is one of the most famous ideas associated with the Greek philosopher Plato. Taking place in a dialogue with Socrates, the allegory describes people chained inside a cave, forced to watch only the images projected by shadows on the cave wall and assuming this is reality. Plato has Socrates assert that a philosopher's role is to bring people out of the cave and into reality by destroying what they assumed by only watching the shadows in the cave.

During the period of the Pre-Socratics (literally, the philosophers before Socrates), many Greek thinkers focused on trying to figure out the nature of the world. They wanted to figure out how one thing (like a human person or even a rock) could be made up of many parts (like the organs, tissues, or even mere molecules involved in the former). When you read the Pre-Socratics, they can seem pretty crazy, but they were on to something, so to speak. They wanted to figure out just what it means for one thing to be many and for a thing to change (but still remain this or that particular thing).

Heraclitus was well known for cryptic sayings. Indeed, he was so well known that a minor American philosopher who used to live in Washington, DC was called the "Heraclitus of the Potomac," meaning that he was as difficult to understand as was Heraclitus.

In any case, Heraclitus is well known for some form of the saying, "You cannot step into the same river twice." One of Heraclitus's insights (among many others) was that change plays an important role in everything that we experience. Thus, when you walk into a river, it has certain molecules. When you step in again, those molecules have gone by and new ones have replaced it. Of course, if we carried this saying to the extreme, we would also say that you and I are quite different people at every moment of our lives—for our molecules change quite a bit.

The great systematizer, Aristotle, would see how this is true—but only in a certain respect. He would do this by analyzing how there are different kinds of causes involved in everything—one of which is the matter (which does indeed change).

In the Apology, the trial of Socrates is recounted as he defends himself against his accusers. The course of his speech, he refers to himself as a "gadfly" sent to wake up Athens. The image of the gadfly is used in Greek mythology, but the point in Socrates's use of the expression is that he was sent (by his supposed "spirit guide" who goaded him into practicing philosophy) to question the Athenians and unseat them of their pride and supposed knowledge. He was a kind of "annoyance" meant to pull them down from their conceits.