After the English Civil War and the Glorious Revolution, many people in Britain sought a way to combat religious divisions and conflicts. John Locke, an early enlightenment figure, set out a theory of religious toleration in his Letters Concerning Toleration, which were published without his knowledge by the letters' recipient.  Locke actually only advocated toleration for any Protestant believers, as he argued both atheists and Catholics were enemies of the English state.

The French philosopher Rene Descartes (1596-1650) is well known for being a leading figure in "rationalism," the philosophy that holds that reason is the chief source of knowledge. Descartes' most famous phrase is almost a perfect summation of this viewpoint. "Cogito, ergo sum," a Latin phrase meaning "I think, therefore I am," asserts in one phrase that existence is proved by a person thinking.

Thomas Hobbes' Leviathan was published in the aftermath of the chaos of the English Civil War. Hobbes began with an extremely negative view of human life and the human condition. His full phrasing of the human condition was in the following quotation: "In such condition there is no place for industry, because the fruit thereof is uncertain, and consequently, not culture of the earth, no navigation, nor the use of commodities that may be imported by sea, no commodious building, no instruments of moving and removing such things as require much force, no knowledge of the face of the earth, no account of time, no arts, no letters, no society, and which is worst of all, continual fear and danger of violent death, and the life of man, solitary, poor, nasty, brutish, and short." (XIII.9)

In his Discourse on Method and his Meditations on First Philosophy, Descartes undertakes a radical reevaluation of everything that he currently believes. He questions everything in order to find out what he actually knows is true. Particularly in the Meditations, he develops a detailed thought experiment in which a potential "evil genius" or evil spirit deceives him—even to the point of making him doubt his senses as well as his very ideas about mathematical facts. Thus, he notices, in the midst of this confusion, that one thing remains—even when he doubts and is potentially deceived, he must at least exist. In order to be fooled, you must actually be. This realization becomes a moment of liberation for him, and he then proceeds to prove that the world does in fact exist, though not before first proving the existence of God.

David Hume is most famously associated with skepticism. Often, people recall the stormy picture that he paints at the end of the first book of his Treatise of Human Nature in which he reflects on the skeptical conclusions at which he seems to arise in the first book's discussions. It is arguable that this is a simplistic reading of Hume. Still, this is what most people know about him from his philosophical writings (especially the Treatise as well as his An Enquiry Concerning Human Understanding). Thus, he is known for doubting the reality of things like personal identity and even the relation of our mind to the external world. 

Lucretius's great poem was a kind of popularization and systematization of the Epicurean cosmology. He presents a kind of worldview that believes that everything physical is merely made up of atoms of different shapes. Indeed, there is nothing more than this matter. He uses this overall cosmology (as had various Epicureans) to present arguments regarding morality.

It should be noted that while this outlook is also called "materialism" (in that it reduces everything to matter), it should not be associated with self-indulgence. Lucretius and other Epicureans were actually quite strict people, whose goal was the reduction of pain, not the maximization of pleasure. Indeed, one of his ways of reducing pain was by critiquing the so-called gods of the Greeks and Romans using the overall atomistic outlook.

The tradition of natural law ethics spans from at least the Stoics through the Middle Ages and beyond. It has earlier resonances in Aristotle as well. Its greatest development occurred in the Middle Ages into early modernity. While the Treatise on Law in the Summa theologiae of Thomas Aquinas is an important text in the natural law tradition, he is actually not the correct answer for this question. He did not write much (if anything) on international law. That really was not a problem in the Latin Middle Ages.

Instead, it is to the Dutch jurist and political philosopher Hugo Grotius that we must turn for such matters. Writing in the midst of many international conflicts in Europe, Grotius penned works like De Jure Belli ac Pacis (On the Law of War and Peace) as well as an important tract on the rights of nations on international waters. He undertook to discuss what rights pertain to people naturally as well as how these are related to the rights of nations. His work was not utterly unique, but it was an important touchpoint for later writers on these matters.

It was George Berkeley who stated esse est percipi. In his Three Dialogues between Hylas and Philonous and A Treatise Concerning the Principles of Human Knowledge, he lays out a scathing critique of John Locke. Berkeley truly believed that Locke's theory of knowledge and metaphysics would lead to skepticism. He tried to overcome this by saying that all of reality is made up of ideas. That is things themselves are ideas and we know those very ideas. This required him to undertake a number of interesting discussions which, although quite strange, present a very interesting set of philosophical musings.

The "Hegelian dialectic" is often rendered as Thesis-Antithesis-Synthesis, meaning any statement can be opposed by an opposite idea, and the clash of the two ideas will create a better philosophical statement. Hegel preferred the terms "abstract," "negative," and "concrete," which better explained the ideas' relationship. Hegel's dialectic was widely influential; it was used by thinkers like Karl Marx and argued against by philosophers such as Søren Kierkegaard and Friedrich Nietzsche.

Immanuel Kant was the most important philosopher of the late eighteenth century. His 1783 work The Critique of Pure Reason established his view that rationality and thought could sufficiently form the basis of morality. In particular, Kant stressed that a prirori, or given as true, knowlegde is all synthetic.