Kierkegaard, Dostoevsky, and Nietzsche, although all nineteenth-century Europeans, were actually quite different thinkers, but all were exemplars of Existentialism because they focused on the individual human and the sense of alienation the individual felt in modern society. In the early twentieth century, authors and philosophers picked up on the common themes these authors highlighted.

Both Friedrich Engels and Karl Marx were German-born anti-capitalists active in London's expatriate radical community. In 1848, they wrote and published The Communist Manifesto as part of a communist organization, originally in German. Their critique of capitalism from a Hegelian viewpoint crystallized reaction to capitalism, and launched the most influential political movement of the twentieth century.

Henry David Thoreau was a nineteenth-century author and philosopher from Massachusetts who was one of the leading figures of the transcendentalism movement. Transcendentalism focused on the goodness of nature, human possibility, and a free-form spirituality that separated itself from Christianity. Thoreau's Walden, a meditation on the two years he spent in a cabin in the woods by Walden Pond, is considered a classic of the genre.

The philosopher Friedrich Nietzsche was an almost revolutionary figure in philosophy during the late nineteenth century. Trained as a scholar of classical philosophy, most of Nietzsche's works, like Thus Spake Zarathustra, The Twilight of the Idols, and Beyond Good and Evil, challenged the philosophical norms of his time. Nietzsche argued against strict rationality, traditional morals, and the view of humanity during the nineteenth century.

Soren Kierkegaard worked in relative obscurity for a giant of Western philosophy, writing only in Danish during the early nineteenth century without a University posting. Although not widely read in his  lifetime, Kierkegaard became hugely influential in the twentieth century and is considered one of the intellectual founders of existentialism. Kierkegaard's focus on individual thought, the subjective nature of human reality, and a critique of traditional religion were all forerunners of twentieth century existential thought.

The work Utilitarianism (1863) was written by John Stuart Mill. He was a prodigiously educated man, taught in a thorough manner by his father James Mill and influenced heavily by Jeremy Bentham. This hard education in his youth actually led to something of a mental breakdown, from which he recovered relatively quickly. The work Utilitarianism is a kind of extension or continuation of Bentham's thought on moral philosophy. It is best known for the principle that states, simply speaking, that the best action is the one that brings about the greatest amount of happiness.

This outlook can become very subjective depending on how one understands it. There remain many philosophers who are utilitarian in bent to this very day, so Mill's work retains great influence in almost all courses on ethics taught in undergraduate institutions.

Hegel is perhaps most famous for having written his dense tome The Phenomenology of Spirit (1807). Ostensibly, this work traces the different forms that human consciousness can take, working through pure sense perception, then to various kinds of scientific, moral, aesthetic, and historical modes of consciousness. The work is, however, an incredibly difficult text to read. The prose is dense and it is often difficult to distinguish between the theoretical and historical claims that Hegel is making in the course of his argumentation.

For all of that difficulty, however, the work was very influential, spawning numerous different schools of interpretation. Although he influenced most thinkers coming after him in continental Europe, his most famous "follower" was Karl Marx. Marx really cannot be considered a direct disciple of Hegel, for he significantly altered his viewpoint. However, he took over the notion of the dialectic of reason exposited by Hegel, using it in his own materialistic philosophy. 

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.

Epistemology, which is a specific form of philosophy, is devoted to the nature of thinking itself, or the theory of knowledge. As such, epistemology deals with questions regarding how people know what they know or acquire knowledge. The field is remarkably new in the history of philosophy, only being coined by the Scottish thinker James Frederick Ferrier in the nineteenth century.