In his work The Groundwork for the Metaphysics of Morals, Immanuel Kant works through in detail what he believes is the very foundational for morality as such. In the course of dense and Teutonic prose, he works out an argument that there is an imperative that applies to all actions and is required for there to be any morality whatsoever. Actually, he believes that there are three forms of this same imperative, the interrelation of which he explains in the course of the Groundwork. These three forms were very influential for the philosophy that came after him and remain the subject of discussion in many philosophical circles to this very day. Put in a simplified form, they are:

• Your reason for acting must be able to be set up as a universal law without implying some kind of contradiction.
• Persons can never be treated as merely means to some end.
• You must act as though you are always a kind of legislator in a kingdom of persons who are ends in themselves, thus making universal laws for action.

Jean-Jacques Rousseau is well known for the opening lines of his The Social Contract: "Man is born free and is everywhere in chains." The work The Social Contract became a mainstay of modern political thought, inspiring revolutionary forms of democratic government. The general problem faced by Rousseau in the text is how it is that a group comes to constitute a political unit freely, constituting a "General Will." His account was significantly influenced by his life in Geneva, Switzerland and always works on a kind of "small scale." While the general notion of social contract is common to many modern forms of governance and statecraft, not every democratic nation formed in modernity owes its origins to Rousseau's thought. (For instance, he was not powerfully influential on American thought.) Still, his work remains an important part of the canon of Western political philosophy and deserves reading by anyone wishing to express a learned opinion on such matters.

The famous Anglophonic, philosophical triad runs: John Locke, George Berkeley, and David Hume. Berkeley famously critiqued Locke's view of knowledge and of mind-independent substances. He held that Locke's overall worldview encouraged us to posit an unknown entity (substance) that never could be known. Only various accidents of that substance (i.e. its various qualities) could be known. For Berkeley, this spelled a disaster—one that led ultimately to a kind of skepticism and atheism. He therefore, proposed that everything is an idea—and that there is no substance. God was the creator of every one of these ideas, thus saving philosophy from atheism—or so he thought.

The Skeptical Scotsman, Hume, believed that Berkeley himself was a father of skepticism. Hume took over Berkeley's ideas and furthered them into a very subjectivistic theory of knowledge, discussed in this Treatise on Human Nature and Essays Concerning Human Understanding. Very often, when these men are listed, they are listed as a triad: Locke, Berkeley, and Hume.

Empirical knowledge is the kind of knowledge that is gained through experience. Strictly speaking, there is no "school of empiricism," but there are general veins of empiricist thought throughout philosophy. Most famous along these lines are various English-speaking thinkers such as Thomas Hobbes, John Locke, George Berkeley, and David Hume. In a strange way, some aspects of the thought of Emmanuel Kant can be added to this list, though he is also heavily influenced by rationalist strands of thought in continental Europe. He does believe in some forms of a priori cognition—that is, knowledge that is independent of experience.

The general idea of an empiricist philosophy is that our knowledge arises from sensation and is limited thereto. It does not hold that we have innate ideas at all. Thus, even ideas like "infinity" are formed by knowing that there could be an infinite sequence of particular experiences constructed from our finite experiences. Of course, each of these thinkers has a unique viewpoint, so "empiricist" is a single category only in a very loose sense.

The most famous account of the mind as a blank slate is that which is expressed by John Locke in his Essay Concerning Human Understanding. There, he emphasizes that everything in the mind must be derived from experience. We are born without any experiences, thus meaning that we are a kind of slate or tablet waiting to be "informed" by our experiences. Locke was no great innovator in this regard. He was taking over an old theme from Medieval and late-scholastic philosophy that was ultimately found in Aristotle. Nevertheless, for many modern thinkers, Locke's theory of knowledge was very influential—even long after it was questioned by many philosophers. (Indeed, it remained influential on the official French curriculum through the early 20th century!)

Gottfried Wilhelm Leibniz wrote a work New Essays on Human Understanding as a response to the work of John Locke. It was not published until many years after the death of both men. In the essay, there is a famous line where Leibniz quotes a so-called maxim of empiricism: Nihil est in intellectu quod non fuerit in sensu—Nothing is in the intellect which was not first in the senses. He then adds a qualifier: excipe, nisi intellectus ipse—except for the intellect itself. The idea is that while all knowledge must be derived from our sense experience, still, in order for there to be intellectual knowledge, there must be an intellect to integrate our experience. Hence, Leibniz hoped to maintain the insights of the general school of thought known as rationalism, which emphasized the direct and pre-experiential role of the human mind in the shaping of our knowledge.