Abstract General Idea - Berkeley argues that Locke's concept of abstract general ideas is incoherent. According to Locke, abstract general ideas are the pieces of our mental geography that correspond to our general terms, such as "man" and "cat" as opposed to "Socrates" and "Garfield". Our general terms refer to these abstract general ideas rather than to anything in the world. According to Locke, we form abstract general ideas by attending to the similarities between particular ideas and abstracting these out (e.g. the similarities between Felix and Garfield yield our idea of cat).
Corpuscular Hypothesis - The Corpuscular Hypotheses was a particular formulation of the new mechanistic science of the 17th century, propounded by Locke's mentor Robert Boyle. According to this theory, matter is composed entirely of tiny, invisible, indivisible bits, called corpuscles. All events and states in the natural world can be explained with reference to the size, shape, and motion of these corpuscles. Locke believed strongly in this view of reality, and it had a large influence on the ideas he expounds in his Essay Concerning Human Understanding.
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?
Empiricism - "Empiricism" is a collective name given to a variety of philosophical doctrines concerned with human knowledge. Empiricists generally believe that knowledge comes exclusively through experience, and that there is no knowledge that human beings are born with. 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.
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 body, the essence of body. Locke attempted to demolish the concept of an essence as any objectively existing thing out in the world. Instead, he claimed that it was only human thought that imposed categories on the world, and thus that it was human thought that makes things the types of things that they are. See also real essence and nominal essence.
Idealism - Idealism, or "immaterialism" as Berkeley calls it, is the view that real objects are mind-dependent ideas See also materialism.
Inference to the Best Explanation - When we use inference to the best explanation, we infer a conclusion by showing that it comprises the best explanation for the evidence under consideration. Inference to the best explanation gives the materialist his best case against skepticism, as Locke showed in his Essay Concerning Human Understanding. Berkeley, however, does not consider this option in his Dialogues.
Innate Ideas - Innate ideas are ideas that are present in the mind at birth. Plato and Rene Descartes were most famous for holding a theory of innate ideas. Locke's first book of the Essay is an attack on the doctrine of innate ideas.
Materialism - "Materialism" is the term Berkeley uses in order to refer to any doctrine or set of doctrines which include the belief in the existence of mind-independent objects.
Metaphysics - The branch of philosophy concerned with asking what there is in the world. Questions about *substance* are metaphysical questions, as are questions about God.
Naïve realism - Naïve realism, also known as "direct realism" is the view that our perception of the world is not mediated by an intervening layer of ideas. Instead, according to naïve realism, we immediately perceive real, material objects.
New Mechanistic Science - Gaining immense popularity in the 17th 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.
Nominal Essence - According to Locke, this is just another name for an abstract general idea. A nominal essence is the set of qualities that men have decided to use in order to pick out a particular type. The nominal essence for gold, for instance, might include qualities like yellow, shiny, malleable. Nominal essences can be relative. A chemist's nominal essence for gold, for instance, might include its atomic number while a lay person's might will not. As a consequence, a piece of metal might count as gold for one person and not for another. Berkeley sought to collapse Locke's distinction between nominal essence and real essence.
Ontology - The branch of philosophy concerned with questions of existence. Ontology is a subcategory of *metaphysics*.
Primary Qualities - Qualities such as size, shape, and motion. According to Locke and Descartes, these qualities really exist out in the world in a way that roughly corresponds to how we perceive them. Berkeley wanted to collapse the distinction between these qualities and secondary qualities.
Rationalism - "Rationalism" is a collective name given to several philosophical systems marked by similar strains. Rationalists tended to believe that reason was extremely powerful, and that by using it we could come to know almost everything that there was to know. The most famous Rationalists were Rene Descartes, Baruch Spinoza, and G. W. Leibniz.
Real Essence - According to Locke, the real essence of an object is the object's microstructure of corpuscles, which gives rise to the observable properties. Berkeley sought to collapse Locke's distinction between real essence and nominal essence.
Secondary Qualities - Secondary Qualities include qualities of color, odor, smell, and taste. According to Locke and 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. Berkeley wanted to put secondary qualities back into real objects, and to thus collapse the distinction between these qualities and primary qualities.
Scholasticism - The dominant school of thought in Western Europe from the Middle Ages through the Age of Enlightenment. Scholastics followed strictly to the doctrines of Aristotle.
Spirit - According to Berkeley, there are only two sorts of things in the world: ideas and the minds that perceive them. These minds are also known as "spirits". Spirits come in two sorts: finite spirits (which include human beings and, Berkeley suggests, angels) and infinite spirit, which is God.
Substance - According to the Scholastics a substance was 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 — God, mind, and Body. Locke grapples with the notion of substance in the Essay, where he mocks both the Scholastic and Cartesian views, but fails to come to any strong conclusions of his own. Berkeley, with his idealism, reduces the sorts of substances in the world once again, banishing matter and keeping only mind and God.
Substratum - In attempting to come up with a theory of *substance*, Locke reluctantly adopts the notion of a substratum as an unknowable, imperceptible, indescribable basis to which all the qualities of a substance belong. The substratum is what those qualities are "of".
Veil of Perception - "Veil of Perception" is a phrase used to refer to the notion that our perception of the world is indirect, filtered through the medium of our ideas. Locke's doctrine of ideas suggests that he subscribes to the veil of perception, though commentators have argued against this reading. Descartes almost certainly believed in this view. Berkeley's idealism was largely a reaction against various skeptical consequences of this view. See also naïve realism.