Dr Tom Kerns
North Seattle Community College



Introduction to John Locke

(1632-1704, age 72)

An Essay Concerning Human Understanding

(published in 1690, at age 58) 

(This lecture is a longish one; you may want to print it out for reading)

John Locke is really a very clear and sensible writer, as you would soon see yourself if you had the time to go look at his Essay Concerning Human Understanding or any other of his philosophical writings. But the Essay is also very lengthy, so I've not assigned you to read any of it this quarter. Instead, I'm going to just tell you a few of the main points he makes in that book. So after some introductory words, the following mini-lecture will be divided into eight of the key points Locke would most like you to learn from his book.


In his youth Locke seriously intended to enter the seminary to become a priest, but in the end he chose to study medicine instead. He became a physician, but practiced only a little, devoting most of his time to writing and official duties.

Locke considers epistemology to be "first philosophy," because he considers it to be the discipline that examines the instrument that does the knowing and philosophizing, viz., the human mind. When you take a biology course, your first lab will be to study and understand the microscope, because the microscope is such a crucial instrument in learning about biology. In a similar way, philosophy's first task should be to examine how the human mind knows anything, which is the task of epistemology. And the first task of epistemology should be to find out if the mind is even capable of knowing anything, and if it is, what are the limits to what it can know.

In his epistemological studies, as we will see below, Locke relies much more heavily on direct sensory experience than on logic and reason. He believes direct experience to be a more reliable source of knowledge than logic and reason. Descartes, a philosopher in the rationalist tradition, chose to rely more on reason and logic for his analyses, but Locke is, after all, a physician. Physicians rely on evidence they get from their patients, they form hypotheses as to what might be the underlying problem with their patient, and they attempt treatments. If the treatments don't work they form another hypothesis and attempt another treatment, and so on. Absolute certitude may be something mathematicians can hope for, but the physician must rely on experience and testing. Locke believes that is also how we derive our knowledge about the world.

In any case, following are some of the key points to learn from Locke's Essay.

I. There are no innate ideas

Locke spends the entirety of Book I in his Essay arguing that human beings have no inborn, or innate, ideas in their minds at birth. Some of the rationalist philosophers wanted to claim that when a human mind comes into the world it already understands such fundamental principles as the principle of non-contradiction - that a thing cannot both be and not be at the same time in the same respect - and the principle that the whole is more than the part. Locke does not believe that we are born with any of these inborn ideas.

Locke believes that when a human mind is first born - whether the birth of that human mind be dated at the moment of parturition (birth), or earlier at the moment of conception, or at some point in between - when the human mind first comes into existence it does not come with any inborn ideas. When it first exists it is a blank slate, a "tabula rasa," an empty surface on which experience will then subsequently write all that we ever know. We may be born with automatic instinctual behaviors - such as automatic reflex motions in response to some stimulus - but these are not ideas or perceptions or what today we might call "contents of consciousness."

II. All ideas come from experience

All contents of consciousness, that is, everything that ever gets into the mind, comes into it from one source only, and that source is experience. Experience is the one source of input into the human mind.

There are two kinds of experience, for Locke.

a. Experience of the outer world, which he terms sensation, and from this mode of experience we derive such notions as blue, round, solid, smooth, heavy, large, etc.

b. Experience of the inner world, which Locke terms reflection, and from this mode we get such notions as fear, love, willing, doubting, affirming, thinking, feeling, believing, remembering, planning, anticipating, and so on.

For Locke, the term "idea" is rather a technical term. By "idea" he means anything that exists in consciousness, i.e., anything that exists in the mind.

So to sum up these first two points, no ideas are inborn in the mind; instead, all ideas, that is, all contents of the mind, come to us ultimately from only one source, experience (either sensation or reflection).

III. Simple and complex ideas

There are basically two sorts of ideas, simple and complex. Simple ideas include all our simple sensory sensations such as red, cold, sweet, loud, soft, round, etc.

Complex ideas are complexes of simple ideas. For example, my sensory experience of a red ball would include a mixture of the simple ideas of red, round, hard, cool, etc. That complex of simple sensations goes into making up my experience of the ball. That is to say, the sensation of this red ball that is in my mind right now, i.e., the "idea" of the ball, derives from the sensory experience of the ball that I am presently experiencing.

In all his thinking about how knowing works, Locke sees himself as following a very common sense way of thinking. He does not see himself as saying anything particularly abstruse or unusual, and certainly not as saying anything that every common sense person wouldn't agree with. In this next section, though, at least toward the end of it, based on common sense though it is, we might at first think he is saying something that common sense should not accept. But you might want to reserve your judgment on that for a while.

IV. Ideas are caused in us by qualities

Ideas (or sensations) in minds are caused by qualities in things. For example, the sensation (or idea) of red in my mind is caused by the quality of red in the thing.

So we now need a definition of what a quality is. Here it is: A quality is a power in a thing to cause an idea in a mind. So a quality is not a thing, really, but is a power in a thing to cause an idea in a mind.

Now this is where it gets interesting. There are two kinds of qualities, according to Locke, primary and secondary. (He actually says there are three kinds of qualities, but we're going to ignore the third kind for now.)

Primary qualities are those that every physical object, every body, must have. Primary qualities are actually in the physical object. There are only six primary qualities:

solidity (bulk)
figure (shape)
extension (size)

Every physical object, in order to even be a physical object, must (according to Locke) have solidity, must have some shape and size, must be either in motion or at rest, must be either one or many, and must have some texture.

Secondary qualities, on the other hand, are nothing but powers in things to produce a sensation in a mind. Now this is an idea that we will need to explore in a bit more detail.

Let's consider the old philosophical question: When a tree falls in the forest and there is no one around to hear it (i.e., no ears of any sort, no squirrels, reindeer, frogs, lemurs or pachyderms), does it make a sound?

Locke's answer would be this. A sound is a sensation in a mind. So "sound" is the kind of existent that exists inside minds, not out in forests. Now the sensation of sound in a mind is caused by a quality out in the world, so let's examine what quality is out there in the world that would cause that sound sensation in your mind.

When a tree falls, it is passing through a medium (air), and in the process of passing through that medium, it causes impact waves in that medium. See the illustration below.

(note: No graphic artists were harmed or mistreated in the production of this image, and the illustration is definitely not drawn to scale.)



Here's what's happening: In the act of falling, the tree passes through a medium (air), and in the process sets up impact waves which move through that medium (much like dropping a pebble in a pool of water causes waves to ripple outward in the medium of water). The impact waves are not themselves sound, but if they strike an auditory apparatus of some sort (like an eardrum-auditory nerve-brain combination) then those waves can cause the sensation of sound ("Boom and Crash") in those brains. So the sound is clearly inside the brain, and what is outside in the world is a quality that has the power to cause a sensation of sound in a brain.

So if there are no brains or hearing apparatuses around, then there are no sounds. In fact, if a tree falls in the forest and there are people there, there is still no sound in the forest. The sound is always in the sensorium of the experiencing brain, not out there in the world. What is out there in the world are impact waves in a medium. Those waves are not themselves sound, but they can cause sound if they impact a hearing apparatus (like the ones for squirrels, lemurs, pachyderms humans, and others).

So sound is never "out there," no matter what sound we're talking about. It is always in a mind. What is out there is some sort of quality which has the power to cause a sensation in a mind.

What Locke would have us notice here is how completely different the impact waves out there in the world are from the sound sensation that exists in a mind. These two realities are totally different kinds of entities. Impact waves are out there moving through air. Sound is a sensation that happens in a mind. Very different kinds of realities.

The concept of the transducer might help us here. Locke did not have transducers around when he was writing, but if he had, he would have loved the idea. A transducer is something in the world of electronics. A transducer is basically any electrical device that changes, or transduces, one form of energy into another form of energy.

A lightbulb, for example, is a transducer. It changes simple electricity (which is not visible) into visible light. It is important to notice here that what goes into the lightbulb (electricity) is not at all the same thing that comes out of it (light). A very naïve person who saw an electric cord leading into the lightbulb might wrongly guess that the cord carried light into the lightbulb. That would be naïve and wrong, of course. In much the same way, what is impacting onto our hearing apparatus is a series of moving waves in a medium. The impact of those waves on our eardrum, and then on into the inner ear, the auditory nerve, and so on, then transduces all that input into a sound. And what Locke would have us notice is that a sound sensation is a very different kind of animal than an impact wave in a medium.

A radio receiver is another example of a transducer. It takes radio-frequency (RF) waves passing through space and transduces them into something we can hear. The RF waves are not themselves hearable until they have been transduced by the receiver.

Our visual apparatus is also a kind of transducer. Light waves come streaming into our eye and impact on the surface of the retina. From that point on our visual apparatus (retina, rods & cones, optical nerve, brain, etc) transduces those light waves into a sensory experience with color, shape, size, etc.

Now we sometimes assume that when any two of us look toward the same object, we must each also be having the same sensory experience. That is, we assume that we each transduce those light waves into the same visual experience. We might even assume that if my dog and I look toward the same object that we will both see the same thing. Or that a lizard and I would see the same thing. But the fact is that different organisms, i.e., different visual apparatuses, in fact transduce things quite differently than humans do.

Many years ago I saw a nice layout in a magazine (unfortunately this was so many years ago - I think I was still in college - that I no longer recall the magazine or date) which showed an array of photographs taken of a single daisy. The interesting thing was that each photograph was taken through the visual apparatus of a different animal. I believe the photographer may have been Lennart Nilsson, or at least was a photographer who used lasers in his photography, and the photographer was able to somehow get the camera to "see" the daisy through the visual apparatus of several different animals. There was a photo of the daisy as seen through the eye of a dog, a lizard, a housefly, and so on. What was interesting was how astonishingly different the daisy appeared through the visual apparatus of those different organisms. In some it was colored certain ways, in others it was colored other ways, in some it was shades of gray with no other colors, in some the shape appeared nothing like a daisy, in some the image was multiplied, and so on. In some cases it appeared so different as to not even be recognizable as a daisy.

It was actually a two-page layout, with these dozen or so images laid out on the left side of the display. Then on the right side of the display was a full page photo of the daisy as seen through the human visual apparatus. The implication of the layout seemed to be, since the photo of the daisy as seen by a human was so much larger, that "This is the way the daisy truly appears, viz., the way it is seen by humans." But of course that implication would not be accurate. The photo of the daisy as seen through the human eyes should have been no larger than the photos through the eyes of all the other organisms.

In any case, that photo layout really brought home to me that we all transduce the stimuli that come into us so that it will register in our brain in categories that our brain is designed to understand. And that every other organism does the same thing.

Now let's bring this back to John Locke and his notion of secondary qualities. This notion that our brains transduce the stimuli that come into them is exactly what Locke meant when he said that secondary qualities are nothing but powers in a thing to produce sensations in a mind.

So Locke's notion is that everything we perceive besides the six primary qualities (solidity, size, shape, texture, number, and motion/rest) are all secondary qualities. They aren't really out there in the world in the way we think they are. Sound isn't really out there in the world in the way we think it is. Nor is color. Nor is temperature, taste, smell, and so on. Neither are other sensations, like pain, for example. We all realize that the pain we experience is not actually out there in the world. We realize that pain is a sensation we have in our brain/mind and that it is caused by something out there in the world (a needle, a burn, etc) which is not itself pain. It is just the cause of pain. We know the same thing about the sensation of tickle, that it is not out there in the world. There may be a feather out there in the world that some cruel person is maliciously applying to us, but we realize that the tickle is purely a sensation that exists in our mind (or brain). Well, Locke just wants us to realize that all our other sensations (except the six primary qualities) are also just in our minds.

V. Judgment constantly alters our perception

This fifth point is simply that there is a crucial difference between "sensation" and "perception" (to put this in contemporary terms). Sensation is simply the raw data that your senses bring into your brain, and perception includes the judgments and interpretations that you add to the sensations so that it ends up having some meaning for you.

Perhaps an example will make this clear.

I once went with a friend to the Chicago Art Institute and on the second floor was an enormous painting which took up one entire wall. It reached clear up to the ceiling and all the way down to the floor. It was an amazingly realistic and full-size painting of a hallway in an art museum, so as you stood looking at it, you had the impression you were looking down a hallway which then opened into another room of the art gallery. You could see some paintings hanging on the wall in the hallway, and one or two people standing looking at them. It was all very realistic, and since it took up an entire wall, with no noticeable frame around it, you could easily believe that you were looking at an actual hallway.

Well, the Art Institute had to put a big piece of plexiglas up in front of this painting because people kept walking into it and bumping it.

The people who walked into the painting were having certain raw sensations when they saw the large painting (simple two-dimensional shapes of various sizes, etc on a flat wall), and to those raw two-dimensional sensations they added certain non-conscious judgments and interpretations about what those shapes and sizes meant. They thus interpreted the two-dimensional shapes to have three-dimensional meanings. They interpreted the lines and colors they saw on the wall to mean that there was open space in front of them.

According to Locke (and to most contemporary sensory psychology too), we do this all the time in all our perceptions. We receive only the raw sensation, but we always overlay that raw sensation with our non-conscious judgments and interpretations, and then what result from that are our perceptions. So the formula would look like this:

sensation + judgment = perception

Or raw sensation, plus how we construe those sensations, yields what our minds then perceive.

VI. Naive realism vs critical realism

Locke's next point is the distinction between naïve realism and critical realism.

First, though, we must distinguish between the philosophy of realism and the philosophy of idealism. Realism is any philosophy which holds that there is a real world out there, and that our sensations and perceptions result from an encounter with that real world. Idealism (as you would seen in George Berkeley if you read him) is any philosophy which holds that there is no real world out there, and that the only things that exist are our perceptions and sensations.

Locke is definitely a realist. He believes that there is a real world out there, and he believes that's what good common sense teaches us too. But Locke wants to distinguish between a naïve realism and a critical realism. Naïve realism would be the unreflective belief that there is a real world out there, and that our perceptions are an exact copy, an exact replica, of what is actually out there in the world. Naïve realism thinks of our senses as more or less like a simple brownie camera, in which we open our senses to the world and the world comes simply streaming into our mind and leaves a perfect copy of itself in our perceptions. Naïve realism would not be aware of how much our senses and brain transduce the information that comes into them.

Locke considers himself to be a critical realist. Critical realism is the belief that yes, there is a real world out there, but our sensations and perceptions are not simple copies of that real world. Critical realism believes that our senses really do transduce and modify the incoming data so that it will register in our minds in ways that make sense to our brains and minds.

Perhaps a metaphor can help this more understandable.

Suppose a classroom full of little laptop binary computers all sitting in a circle and having a discussion amongst themselves. All any of them can perceive, of course, are ones and zeros, since they are all binary computers and that is all that binary computers can perceive. (If we want to communicate with our computers, you know that we must translate everything we want to say to them into ones and zeros; so if I want to say "L" to my computer, I have to translate that L into 10001010 so that the computer can understand it, because strings of ones and zeros are all that its little processor can register.)

In any case, imagine all these little computers thinking about what the real world out there must actually be like. The naïve realists among them would believe that the world consists of nothing but ones and zeros, because that is exactly what they all see. The critical realists among them would believe that yes, there must be a real world out there, but it probably does not consist of only ones and zeros. "The world comes streaming into us through our input devices," says one critical realist computer," but our input devices transduce the world into the categories that we can understand, which is ones and zeros. The world out there is not actually ones and zeros, but ones and zeros are the only categories we have for perceiving the world, so if we're going to see anything at all, it will have to register with us in those categories."

Locke believes that we are just like the binary computers, except that we have a few more categories of perception than they do. (Later we will see that Immanuel Kant believes that we have twelve or fourteen categories of perception.)

To sum up, naive realism believes there is an isomorphic relationship between what we perceive and what is actually out there in the world. You don't have to learn much sensory psychology, though, before you discover that Locke's critical realism is probably a lot closer to the mark

VII. Four kinds of existents

Locke believes that there are only four kinds of realities (existents) that exist in the world.

1. He believes there are selves (or minds). He believes that we know about the existence of minds ­ both our own and those of other people -- by a process he terms "intuiting."

2. The second kind of existent Locke calls ideas, i.e., the contents of minds. He believes we know about the existence of ideas by reflection.

3. The third kind of existent is things, or physical objects. Locke believes that we know about the existence of things through sensation.

4. Fourthly, Locke believes that there is a God, and that we know about God's existence by logical proofs for his existence. (We will not be looking at any of the classical arguments for the existence of God, but if you ever take a Philosophy of Religion course you will learn about them there.)

VIII. Two big questions

Finally, there are two key questions Locke considers it important to ask, and it may even be that the asking of these two questions is more important than the specific answers Locke gives to them.

1. The first question is this: Can we know that things continue to exist during the intervals that they are not being observed by anyone? For example, imagine that you are perceiving the mug on your desk, but that you then turn away from it so that you are no longer perceiving it in any way (and neither is anyone else). You do not see it, feel it, hear it, smell it, or perceive it in any way (and neither does anyone else). Then you turn back and perceive it again. Then you turn away and are no longer perceiving it. The question is this: during the intervals that the mug is not being perceived, can we know that it continues to exist during those intervals?

This may seem like a silly question, but when one is doing epistemology it is important to examine closely the assumptions that are made about what it is possible to know. And Locke is willing to examine this assumption we all seem to have, that things somehow continue in existence even though no one is perceiving them. So that's question number one.

2. The second question is this: Even during the times that we are directly observing an object can we know for sure that the object actually exists.

Now the answers that Locke himself proposed for these questions are not nearly as important as the fact that he posed the questions. The next two philosophers who follow Locke in the empiricist tradition - George Berkeley and David Hume - take these two questions very seriously and suggest answers very different from those that Locke offered.

Just for the record, though it is not particularly important for our purposes here, here are the answers that Locke proposed for these two questions.

1. Can we know that objects continue to exist even when they are not being perceived by anyone? Answer: Well, perhaps we cannot be absolutely certain of their continued existence during the times when they are not being perceived, but common sense tells us that in all probability they do continue to exist even when not being perceived.

2. And can we know that objects exist even when they are being perceived? Locke's answer: Surely no one would be so skeptical as to hold that we cannot know objects exist when they are being directly perceived. Common sense tells us that of course we can know that objects exist during the intervals that we are directly perceiving them.


You might now ask yourself whether you think Locke's answers to these two questions should be considered adequate or not.

George Berkeley, while he greatly admired Locke and his work, did not consider that Locke had carried his epistemological examinations quite far enough. I sometimes wonder if Berkeley may have thought that Locke lost a bit of his question-asking courage here when he caved in to too-simple answers to these two questions.

So one of the big questions Berkeley takes up when he writes both his Principles of Human Knowledge and his Three Dialogues Between Hylas and Philonous is the even more central question: What does "to exist" actually mean? We may assume that we understand this simple word which we use so easily and frequently, but what exactly are we claiming when we claim that a thing "exists?"

Berkeley's answer to this question is a most interesting one.