Heraclitus believed that everything in the universe is in a constant state of flux and change.

Epicurus believed that the goal of life is to achieve a state of tranquility and freedom from mental disturbance, which he called ataraxia.

Aristotle argued that the highest good for human beings is to live in accordance with virtue, which he defined as a state of character that enables us to flourish as human beings.

Augustine believed that God is the source of all truth and goodness, and that the goal of human life is to seek union with God.

Descartes argued that the only thing he could be certain of was his own existence, which he expressed in the famous phrase, 'Cogito ergo sum' (I think, therefore I am).

Locke argued that people have natural rights, that government is based on the consent of the governed, and that the separation of powers is essential for a just government.

Hume argued that all knowledge is derived from experience, that there is no such thing as substance, and that cause and effect are merely constant conjunctions.

Kant argued that the mind imposes its own categories on the world, that there are things-in-themselves that we cannot know, and that the world is fundamentally unknowable.

Hegel argued that history is a process of dialectical development, that the individual is a product of their social and historical context, and that the state is the highest form of human community.

Marx argued that the labor theory of value, the exploitation of workers under capitalism, and the inevitability of class struggle are all essential features of capitalism.

Nietzsche argued that the death of God, the Übermensch (Superman), and the will to power are all essential features of the modern world.

Heidegger argued that Dasein (being-there), temporality, and being-towards-death are all essential features of human existence.

Sartre argued that existence precedes essence, that freedom is the defining characteristic of human beings, and that bad faith is a fundamental aspect of human existence.

Camus argued that life is absurd, that it is important to live in the present, and that we need to find meaning in a meaningless world.

Arendt argued that labor, work, and action are essential aspects of human existence, that the public realm is essential for human flourishing, and that totalitarianism is a threat to human freedom.