Abstract General Idea

A 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). Locke identifies abstract general ideas with what the Scholastics and Cartesians would call essence.

Argument from Parsimony

In an argument from parsimony, the conclusion rests on the suppressed premise that it is best to posit as few existents in the world as possible. Rather than prove conclusively that something does not exist, an argument from parsimony just shows that any puzzle that might be solved by positing the existence of the thing in question can also be solved (better) without positing the thing's existence. There is no need, therefore, to posit the thing's existence, and so, by the suppressed premise, we should not. Locke's argument against the existence of secondary qualities in the world is an argument of this form.


Followers of René Descartes. See also rationalists.

Corpuscular Hypothesis

The Corpuscular Hypothesis 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 powerful influence on the ideas he expounds in his Essay.


Demonstration is the middle grade of knowledge according to Locke, not as good as Intuition but still a legitimate form of knowledge. Demonstration is knowledge that proceeds by reasoning out a proof. Each step of the proof in demonstration must be an intuition, so demonstrative knowledge depends upon intuitive knowledge.


"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 human beings are born completely without knowledge. 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.


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?


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 would be 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. Locke attempted to demolish the concept of an essence as anything objectively existing 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.

Innate ideas

Innate ideas are ideas that are present in the mind at birth. Plato and René Descartes are most famous for holding a theory of innate ideas. Locke's first book of the Essay Concerning Human Understading is an attack on the doctrine of innate ideas.


Intuition is the highest grade of knowledge according to Locke. In intuition, the mind perceives the connections between ideas as soon as the ideas are understood. Examples of intuitive knowledge would be the knowledge that I exist and the knowledge that A = A. Intuitive knowledge is much the same thing as what later philosophers would call analytic truths.


The branch of philosophy concerned with asking what there is in the world. Questions about substance are metaphysical questions, as are questions about God.


According to Descartes, a mode was a way of being a certain substance (e.g. square or red). Locke expands considerably upon this definition. For Locke, a mode is something that depends on substances for its existence, something that cannot exist independently. Properties such as square and red are modes for Locke, but so are numbers, and more abstract concepts such as "gratitude," "beauty," and "hour."

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

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, or malleable. Nominal essences can be relative. A chemist's nominal essence for gold, for instance, might include its atomic number while a layperson's might will not. As a consequence, a piece of metal might count as gold for one person and not for another. See also real essence.


The branch of philosophy concerned with questions of existence. Ontology is a subcategory of metaphysics.


According to Locke, most of the facts of science and everyday life are correctly classified under this heading. Opinion differs from knowledge in that it is only probabilistic and not certain.

Primary qualities

Qualities such as size, shape, and motion. According to Locke these qualities really exist in the external world in a way that roughly corresponds to how we perceive them. See also Secondary Qualities.

Rationalism (Rationalists)

"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 are 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. Although these microstructures do exist out in the world, real essences do not generate natural kinds the way the Scholastics and Descartes thought that essences did. This distinction arises because Locke held that the determination of what part of the microstructure is included in a thing's real essence is based wholly on the man-made nominal essence.


The dominant school of thought in Western Europe from the Middle Ages through the Age of Enlightenment. Scholastics strictly followed the doctrines of Aristotle.

Secondary Qualities

Secondary Qualities include qualities of color, odor, smell, and taste. According to Locke, there is nothing in the world that corresponds 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.

Sensitive knowledge

The lowest grade of knowledge according to Locke, it does not even count as a wholly legitimate form, but is more akin to pseudo-knowledge. Sensitive knowledge is our knowledge that there is an external world corresponding roughly to our perception of it.


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 Book II of the Essay, where he mocks both the Scholastic and Cartesian views, but fails to come to any strong conclusions of his own.


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."

Transparency of the Mental

"Transparency of the Mental" is a phrase used to describe Locke's assertion, espoused in Books 1 and 2 of the Essay, that nothing can be in the mind without our being aware of it. The assertion is based on Locke's identification of thought with consciousness.

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.