A posteriori - An a posteriori truth is a truth that is arrived at by observing the world. An a posteriori fact is arrived at through a posteriori reasoning (reasoning that involves facts observed in the world). For example, the fact that John has blonde hair would be an a posteriori truth, based on a posteriori reasoning. The fact that heat is molecular motion would be another such fact. Many philosophers claim that all substantive facts about the world are a posteriori.
A priori - An a priori truth is one that can be arrived at without any observations of the world. A priori reasoning looks only to logical connections between ideas. For example, the fact that all bachelors are unmarried is an a priori truth. In order to determine that this claim is correct you do not need to go out into the world and survey all bachelors. Rather, so long as you understand the meaning of the words involved, you know that the claim is true. Many philosophers believe that all a priori truths are tautologies like "all bachelors are unmarried." Though the term was not yet in use during Descartes' lifetime, Descartes would be an example of a philosopher who believed that a priori reasoning could produce substantive claims about the way the world is.
Clear and Distinct Perception - Clear and distinct perceptions are defined by Descartes as those perceptions which are so self-evident that, while they are held in the mind, they cannot logically be doubted. Examples of clear and distinct perceptions include the propositions "A = A" and "I exist." All knowledge, according to Descartes, is supposed to proceed from clear and distinct perceptions; no proposition is supposed to be judged as true unless it is perceived clearly and distinctly.
Cogito Ergo Sum - "Cogito Ergo Sum" is the Latin translation of Descartes' famous statement "I think, therefore I am." Often referred to as the "cogito" for short, this is the first certain truth that Descartes hits upon in Part I of the Principles.
Empiricism - "Empiricism" is a collective name given to a variety of philosophical doctrines concerned with human knowledge. Empiricists generally believe that substantive knowledge requires experience and that there is no knowledge with which human beings are born. In addition to John Locke, some famous empiricists have been George Berkeley, Thomas Reid, David Hume, Rudolph Carnap, G.E. Moore, and W.V. Quine.
Epistemology - The branch of philosophy concerned with knowledge, belief, and thought. Epistemological questions include: What is knowledge? How do we form beliefs based on evidence? Can we know anything?
Essence - An important concept in Scholastic philosophy, an essence was supposed to be the quality that made something the type of thing that it was. The essence of man, for example, was believed to be rational thought because it is rational thought that distinguishes man from all other beings. The essence of a knife was the ability to cut. Descartes tried to demonstrate that there are only two essences in the world—thought, the essence of mind; and extension, the essence of body.
Extension - Extension is the principal attribute of body. To be extended means to have length, breadth, depth, or height.
Formal Reality - Formal reality is simply the reality something has in virtue of existing. It is garden variety, normal reality. Formal reality comes in three grades: finite, infinite, and mode. Only God has infinite reality. All substances have finite reality. All qualities have modal reality. The concept of formal reality is crucial to Descartes' causal argument for the existence of God. See also Objective Reality.
Innate Idea - An innate idea is an idea that is present in the mind at birth. Descartes believed that inborn in our minds are certain mathematical ideas (such as the ideas of geometrical shapes), metaphysical ideas (such as the idea of God and of essences), and eternal truths (such as the truth that something cannot come from nothing). These innate ideas play a central role in his theory of knowledge.
Mode - According to Descartes a mode, is a determinate way of being a principal attribute. All modes of body are determinate ways of being extended. Examples of modes of body would include squareness, being two inches by two inches by two inches, being unified. All modes of mind are determinate ways of being thought, e.g. imagining a unicorn, believing I will have steak for dinner tonight, wishing you would go away.
New Mechanistic Science - Gaining immense popularity in the seventeenth century, this movement sought to replace the messy and complicated Scholastic model of the world with a simpler picture. According to the mechanistic view, all explanation can be given in terms of the principles of matter and motion. Within the mechanistic camp, there were a wide variety of competing theories regarding what those principles should be. Descartes' was one among those theories.
Objective Reality - Something has objective reality in virtue of representing something else. Descartes applies objective reality only to ideas and does not say whether other representational entities, such as paintings, have objective reality. The amount of objective reality an idea has is determined solely on the basis of the amount of formal reality contained in the thing being represented. An idea of God has infinite objective reality; an idea of your cousin, assuming you have one, has finite objective reality; and idea of red has modal objective reality. The concept of objective reality is crucial to Descartes' causal argument for the existence of God. See also Formal Reality.
Ontology - The branch of philosophy concerned with questions of existence. Ontology is a subcategory of metaphysics.
Ontological Argument for God's Existence - An ontological argument for God's existence is an argument that argues for the conclusion that God exists by claiming that existence belongs to the very idea of God. Though Descartes makes an argument of this sort, he is far from the first to do so—the medieval philosopher St. Anselm made the most famous formulation of the ontological argument. Even Plato seems to make an argument of this type in the Phaedo. The popularity of ontological arguments decreased dramatically when Immanuel Kant showed that they involve a fatal logical fallacy; they treat the existential verb (to be) as a property like other properties—a property that something can either have or not have. In fact, existence is quite unique as a property, since if something does not exist it does not "have" or "not have" anything. It just is not.
Plenum - Descartes believed that space was entirely filled with body and, thus, was more accurately described as a plenum rather than as a vacuum.
Primary Qualities - Qualities such as size, shape, and motion. According to Descartes these qualities really exist out in the world in a way that roughly corresponds to how we perceive them. See also Secondary Qualities.
Principal Attribute - According to Descartes, every substance has a principal attribute that determines what that substance is. Since there are only two substances in the world, mind and body, there are only two correlating principal attributes, thought and extension. The link between a substance and its principal attribute is extremely strong. A substance cannot exist or cannot even be conceived of without its principal attribute. A body without extension, or a mind without thought, is logically incoherent.
Rationalism - "Rationalism" is a collective name given to several philosophical systems marked by similar strains. Rationalists tend to believe that reason is extremely powerful and that by using it we can come to know almost everything that there is to know. The most famous rationalists were Rene Descartes, Baruch Spinoza, and G. W. Leibniz.
Secondary Qualities - Secondary qualities include the qualities of color, odor, smell, taste, heat, cold, pain, pleasure. According to Descartes, there is nothing in the world corresponding to our ideas of these qualities. What we see as "red", for instance, is really just a colorless arrangement of corpuscles, which, by their particular size, shape, and motion, have the power to produce in us the sensation of redness.
Scholasticism - The dominant school of thought in Western Europe from the Middle Ages through the Age of Enlightenment. Scholastics were primarily concerned with working out problems in, and extending the theories of, Aristotle.
Substance - According to the Scholastics, a substance is the most basic unit of existence. Descartes agreed, but he reduced the types of substances in the world from an innumerable mass to only three—body, mind, and God (a variation on the substance of mind).
Thought - Thought is the principal attribute of mind. Descartes' definition of thought is fairly wide. It includes all mental operations, such as imagining, sensing, reasoning, believing, hoping, doubting, wishing, willing, and etc. There is some dispute over the criteria of thought. Many philosophers feel that Descartes believed that consciousness was the mark of thought. Others hold that Descartes defined thought as anything representational. Still, others hold that Descartes believed that thought was determined by the combination of these two criteria.