The Nature of Ideas


A Brief Introduction to Epistemology

By Jonathan Dolhenty, Ph.D.


Ideas are the building blocks of knowledge. Judgments, which are expressed in sentences called propositions, are made up of ideas. A judgment is an act of the mind pronouncing the agreement or disagreement of ideas among themselves. We need, therefore, to direct our attention to what ideas are and how are they formed.

The Formation of Ideas

We can begin to understand the nature of the idea by taking a look at the process we use in forming ideas. The process which is described here is a philosophical process, from the viewpoint of common sense critically examined, and not one described in the terms of the physical and biological sciences. So pay particular attention to the definition of the words used.

How We Form Ideas

All knowledge begins with the senses. We perceive many objects out there in the world which are presented to our various senses. For example, we see a lump of sugar and say it is white in color. We touch it with our fingers and notice it is granular in texture. We taste it and discover it is sweet. This combination of white, granular, and sweet forms a sense image in us which is retained in our imagination and memory. We can recall this image even when the object is not present before us. If we hear the word “sugar,” we can recall the image of sugar. (Consult the chart below on Sense Knowledge.)

The same thing applies to other words in a similar way. When we hear words like “mountain,” “dog,” “John Wayne,” and so forth, we can recall their image as long as we have perceived them in fact or have heard or read about them. When my little dog, Ming, hears the word “treat” he immediately begins to jump and bark. This is the word he has associated with the little dog bones he gets now and then as a special favor. By means of his memory and imagination, he recalls the image of the bones that he has previously experienced.

This sense image of the imagination is the first step in the formation of an idea.

Now that the sense image is in the imagination, the mind begins to think and the intellect gives its attention to the sense image in order to make its own representation of the object. It does this by a process called abstraction. Let’s take an example of this process in action so this matter will be clear.

Through experience we come into contact with objects which we designate by the word “dog.” One thing we discover right away is that there are a great many differences among the dogs we experience. Some are large, some are small. Some are fat, some are thin. Some are black, some are brown, and some are another color. Some are old, some are young. Some are male dogs, some are female dogs.

The intellect notices these differences and discovers that some of these characteristics change or disappear, while others take their place. But the intellect also realizes that some characteristics remain intact throughout all the changes. For instance, all dogs have a body, take in nourishment and water, and have physical senses enabling them to experience the world around them. The intellect notices that dogs are living, sentient, and conscious substances. The word “substances” means a thing that exists in itself. Dogs are substances, as are human beings, desks, rocks, plants, and so forth.

It doesn’t matter what color a dog is or whether the dog is large or small or has a mean temperament. There are some things that make a dog to be a dog and those things we call the essential elements which constitute his nature. A dog must be sentient to be a dog. This is a characteristic essential to be a dog. A dog is a living substance made from chemical materials. These are essential to be a dog.

Once the sense image is formed and registered in the imagination and memory, the intellect strips the individual dog of all the nonessential characteristics, retains the essential characteristics only, forming them into what is called an intellectual image. The intellectual image of a dog is that it is a sentient, living, bodily substance. This intellectual image of a dog is the idea of dog.

We form the idea of “man” or “human being” the same way. Our experience brings us into contact with many beings which we designate by the word “man.” We notice differences among these many beings but we also recognize some similarities. When a particular human being is sensed, a sense image is formed and from this sense image the intellect abstracts the essential characteristics, forming an intellectual image or idea of what is necessary to be a “man” or “human being.”

Human beings differ in many nonessential characteristics. Just like dogs, some human beings are tall or short, fat or thin, white or black or brown, healthy or diseased, and so on. But what is it that all human beings have in common? What are the essential characteristics to be a human being? Certain characteristics are obvious. Human beings are a substance, that is, an independently existing being, and they are made of living material and are sentient. All human beings must have at least these essential characteristics to be human. But dogs and other animals also have these characteristics. So how do we differentiate human beings from other animals?

There is one essential characteristic that human beings possess that animals do not. (It should be noted here that while human beings share certain essential characteristics with other animals, human beings here are considered to be different in kind from other animals.) This essential characteristic is rationality. Rationality refers to the ability to form abstract ideas, to reason intellectually, to form moral judgments, and so on. So far as we know, animals do not possess these abilities.

The essential characteristics of a human being are that of a substance that is material, living, sentient, and rational. Since animals are also substances that are material, living, and sentient, and the only difference between animals and human beings is the essential characteristic of rationality, philosophers have traditionally referred to the human being as a rational animal.

Let’s consider plants for a minute. We experience a variety of plants we call “trees.” How do we form the idea of a tree? Once the sense images of these plants is available to our intellect, it ignores the differences among them as to size, shape, color, and other nonessential characteristics. The intellect retains the essential elements, those things that are found in all trees, and combines them into a single intellectual image or idea of “tree.”

We are now ready to define the word “idea.” An idea is the intellectual image or representation of a thing. Another word for “idea” is “concept.”

Idea and Sense Image

The idea must not be confused with the sense image. The sense image is formed in the imagination as a result of sense data. We see, hear, feel, smell, or taste something and form a sense image. The idea, on the other hand, is formed in the intellect.

The sense image is always concrete, individual, particular. We cannot form a sense image of all members of a class of individual objects. Our imagination can make a general picture of a black dog, for instance, but it cannot make a sense image that will fit a black dog and a white dog at the same time. The sense image will fit either a black dog (a particular) or a white dog (another particular) but not both.

No single sense image of “man” or “human being” can adequately fit at the same time a baby, a teenager, an old woman, Thomas Jefferson, or Socrates. No single sense image of “animal” can fit an amoeba, a spider, an elephant, a salmon, and a dinosaur. The reason for this should be clear. The senses picture single, concrete, particularized objects. The imagination makes the sense images from these sense pictures. So the sense image must also be concrete, individual, and particular.

The idea or concept is different. The idea can apply equally to an individual and to a class of individuals and even to a number of classes of individuals. We say the idea is universal. The idea “human being” can fit a baby, a teenager, an old woman, Thomas Jefferson, and Socrates. No problem. The idea “dog” can fit a German shepherd, a dachshund, a great Dane, and even my little Lhasa Apso, Ming. No problem. The idea of “human being” and “dog” can apply to a single individual of the class or to the entire class of individuals itself.

Another difference between the sense image and the idea is that the sense image becomes very vague and indistinct when it becomes complex and has to deal with a multitude of detail. We can form a sense image of five people in a row. But to imagine five million in a row is virtually impossible. The idea of five million people, however, presents no problems and is as clear as the idea of five people.

Still another difference between the sense image and the idea is that we can have a very clear idea of some things for which we cannot reasonably form a sense image. For example, we often talk about such things as the “law,” or virtue, or life, or justice, or, for that matter, logic. How does one form a sense image of such things? We do not even attempt to do so. We simply think and talk about virtue, justice, logic, and so forth, without worrying about a sense image of them.

It is important to realize that the imagination and intellect, the sense image and the idea, are in close harmony. The intellect wouldn’t have anything in it if it were not for the senses. The intellect is dependent on the imagination to furnish it with materials for ideas. All knowledge begins in the senses and there is nothing in the intellect that at one time was not derived in some form from sense data. Ideas are totally distinct from sense images yet dependent on them. (Consult the chart below on The Process of Abstraction.)


Unifying SenseThe power to co-ordinate the different aspects of objects brought in by the external senses
MemoryThe power to store up and recall past objects and states of consciousness and recognize them as having been present in former experiences
ImaginationThe power to form mental images of perceived objects and the ability to reproduce these images even when the perceived objects are absent
Estimative SenseThe power to perceive the useful or harmful character of particular things for our well-being

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The external object sends forth certain forms of energy which are received by the senses. At this point we are perceiving objects and are said to experience perceptions.
Sight – Touch – Taste – Hearing – Smell
IMAGINATIONSense image stored as a trace or disposition. Can be later reviewed.
MEMORYSense image stored as to date and localizes past experiences.


This is a total sensory impression and a completed cognitional image. It is psychical or mental representation of the external thing. It is concrete and individualized.
The essential elements of the object are abstracted from the concrete conditions in which they are found in the sense image.
These abstracted elements are presented to the intellect.
The intellect goes into action and mentally expresses this intelligible content in conceptual terms. It forms an idea or concept in an abstract and universal manner. The concept is then stored away in the intellectual memory for future recall and identification.

Dr. Jonathan Dolhenty was the Founder and President of The Center for Applied Philosophy and the Radical Academy, and is Honorary Philosophy Editor at Self-Educated American. Self-Educated American has adopted these projects beginning with a republishing and preserving of all of Dr. Dolhenty’s work.