Arguably the foundation of the Enlightenment, the belief that, by using the power of reason, humans could arrive at truth and improve human life.


A philosophical movement that emerged in response to Rationalism and maintained that human perception is too relative to be considered credible. For Descartes his practice of skepticism meant that to prove anything beyond a shadow of a doubt he had to call everything into doubt. This practice then involved critically examining one’s own knowledge and perception to determine whether they are true. But skeptics also must ask whether there is such a thing as true knowledge. In other words, they must ask whether it is possible to know anything for certain. It should be noted that Descartes was not the first person to employ skepticism—the tradition reaches back as far as the history of philosophy.

Cogito ergo sum

Latin phrase that translates to, “I think, therefore I am.” Arguably the most famous single phrase in all philosophy and is generally considered the starting point for modern Western philosophy. In it, the Meditator finds his first grip on certainty after the radical skepticism he posited in the First Meditation. The cogito presents a picture of the world and of knowledge in which the mind is something that can know itself better than it can know anything else. The idea that we know our mind first and foremost has had a hypnotic hold on Western philosophy ever since, and how the mind can connect with reality has ever since been a major concern. In this conception, the mind ceases to be something that helps us know about the world and becomes something inside which we are locked.

Sum res cogitans

An argument advanced by the Meditator that claims that he (man) is essentially a thing that thinks.


For Descartes, as for most thinkers of his time, substances are the fundamental building blocks of reality. Substances can exist independently and are indestructible. In Cartesian Ontology, there are two kinds of substances: bodies and minds. Of substances that are minds there are finite minds, such as people and angels, and there is the infinite mind, God. Thus, when the Meditator asserts the sum res cogitans, he is asserting that, as a substance, he is a mind rather than a body.

Primary Attribute

Attributes that explain the essence of the substance they belong to. According to Descartes, the primary attribute of the body is extension and the primary attribute of the mind is thought. 

Omni-generic Attributes

Attributes that can hold of any substance and do not define their essence. Examples are existence, duration, and numbers.


Modifications of primary attributes, modes are ways in which something can have a certain primary attribute. For instance, squareness is a mode of extension since it is a way in which a body might be extended. 

Formal Reality & Objective Reality

Formal reality refers to the kind of reality things have in this world and objective reality is the reality of the objects represented by different ideas. Thus, an idea can have formal reality, being a mode of thought itself, and it can also have objective reality, representing something outside of itself. 


The process by which physical energy from objects in the world or in the body stimulates the sense organs.


The process by which the brain interprets and organizes sensory information.

Act of Deception

An act of falsity, which deals with what is not. 


The intellect is the faculty that not only understands and thinks, but also senses and imagines. All these are value-neutral acts in themselves. The intellect is finite and limited because there are varying degrees at which perceptions and understanding can function.

The Will

The will is responsible for affirming and denying, and it is in the will that value and the possibility for error manifests itself. The will is not finite because its efficacy is not a matter of degree. 


A philosophical term whose currency has waned since the 17th century. The essence of a thing is the property or set of properties that the thing cannot do without. For instance, Aristotle defines rationality as an essential characteristic of a human being. A person could lose a leg and still be human, but a person could not cease to be rational and remain human.

Aristotelian Empiricism

According to Aristotle, we learn the essence of, for example, a triangle, by examining instances of triangular-shaped objects in the world and extracting the essence of triangles from these worldly instances.

Sensory Perception & Intellectual Perception

Sensory perception is perception using the imagination, while intellectual perception uses understanding. 

Primary Qualities & Secondary Qualities

Primary qualities are properties such as heat, color, and taste, while secondary qualities include size, shape, and texture.


The idea that secondary qualities exist exclusively in the mind and not in any way in bodies. Secondary qualities do not represent anything in the corporeal world, according to this interpretation, though they may be caused by things in the world. Sensationalism implies that when one perceives red, the mind is, in some sense, red.


The idea that secondary qualities exist both in the body and the mind, but in very different ways. Colors, for instance, manifest themselves in bodies as surface textures which reflect light. The thrust of the physicalist argument is not that secondary qualities are present in the bodies themselves, but rather that these surface textures are what cause color sensations to be present in the mind. 


Extended is a word that Descartes uses to describe something like “has physical mass” or “takes up space.” Therefore, if we so clearly perceive that bodies are extended, that extension must be an essential part of bodies. 

Popular pages: Meditations on First Philosophy