I will say just a few words about Plato's life and the influences on his writing. His first calling in life was dramatic poetry, and this background follows through into his dialogues.

Socrates influenced many great thinkers. I offer just a few highlights of his life and thought which completely altered the development of philosophy.

Here are some thoughts on Aristotle's approach to the idea of soul or psyche. It explains our unity, but also touches on our nous/intellect and how we grasp the world.

Aristotle floated a preliminary idea about a primary substance in the Categories. He suggested the primary substance could be the individual such as Socrates because all the other categories depend on that individual thing for their existence. But after introducing the idea of matter and the other causes, he sees a problem. The individual thing cannot be the primary substance. It must be the species-form.

Aristotle introduces four causes (explanations) for change: material, formal, efficient, and final. These are easy to understand when applied to material constructions, but it is essential to understand how he applies them to living things.

In section 2 of his Grioundwork, Kant expands on some earlier themes, and introduces the idea of the Categorical Imperative.

Kant takes the discussion of morality in a new direction, and challenges some central ideas of the Utilitarians. Happiness cannot be a guide to moral action. The only purely good thing in the world is a good will.

David Hume's empiricism rattled the bones of Kant. Hume's attack on causality and ethics pushed Kant to respond with two books: A Critique of Pure Reason & A Groundwork of a Metaphysics of Morals.

Ethical relativism seems to pop up regularly in classes on ethical topics. It can become a huge distraction or digression from the topics at hand. I think it is ultimately a waste of time. If you are interested in following some of the arguments about relativism I suggest a look at these two short books:

A Refutation of Moral Relativism by Peter Kreeft

Ethics: A Very Short Introduction by Simon Blackburn

Bentham's Hedonic Utilitarianism did not distinguish types of pleasures, and so faced an attack that it was a pig's philosophy. A pig would be happier than Socrates. Mill suggests an answer to this attack: Eudiamonia. We need to aspire to higher pleasures for a good life.