This phrase, "subjective experience" should be regarded as one two-word term, because there is not going to be a corresponding concept of "objective experience" or "non-subjective experience." The two words ("subjective" and "experience") are being used together to designate one thing, only because the two words used together are most helpful in conveying the meaning intended, as will be seen in what follows.

There is a metaphor that I find useful in increasing our understanding of these issues. I ask that you imagine a cartoon that shows two people looking at a house from two different directions. Over the head of each of the people is a "balloon" of the sort used in cartoons to depict either speech or internal thought. In our metaphoric cartoon, there is a house in each of the two balloons, in addition to the house they are both looking at. What is in each balloon represents the subjective experience of the house for each person. Each of those three houses (two in the balloons and one not) looks different to us as we look at the cartoon. (This difference is well recognized as the difference in perspective). Each person in the cartoon knows only the house that is in his/her own balloon, and it is not the same as the house we see them both looking at. So each of them understands that what is in his/her balloon is different from what is in the other's balloon. Subjective experience of something by two or more people does not have to be the same. And this is true for more than one reason. The reason given in this example is the difference in perspective.

Now, to examine another reason, we first must notice that we can say that what is in the balloon, that is, how the person is experiencing the house, has to be different from the actual house. (After all, the two houses in the balloons are different from each other, so they can't both be the same as something else, the actual house.) In fact, there is nothing about the "actual house" that the person can "directly" experience, in that whatever the house consists of has to be converted, or changed (for example, through light reflected off the house, transformation of that light into electrochemical reactions of the retinal receptor cells, becoming in turn electrochemical processes involved in conduction of nerve impulses along axons, etc.) ultimately into that subjective experience.

To make this fact even clearer, let us do a thought experiment that involves our subject looking at a chair. Let us make the assumption that with some sort of very advanced technology we can exactly reproduce in that subject's brain the exact same state of affairs as is occurring at present. So, at present, our subject is looking at the chair. Now, a few moments later, we do one of two things. Either we take the chair away but reproduce in our subject's brain the exact same state of affairs, or we leave the chair but remove the subject's brain (cause it to suddenly die, for instance). Under which set of circumstances would the subjective experience of the chair be reproduced? So what is necessary to produce that subjective experience, the chair or the brain? Upon what does that subjective experience depend, the structure of the chair or the structure and functioning of the brain? What is there, then, of the actual chair that is part of the subject's subjective experience of the chair? It should be clear that the actual chair, independent of the subjective experiencing of it, is not in any way the same as the subjective experience of the chair, which is all that someone can "know." (We will later, however, consider the "relationship" between the "actual chair" and the subjective experiencing of it.)

This fact, that one's "internal," or subjective, experience is not the same as "reality" (i.e., that which exists independently of anyone experiencing it), has long been recognized. One way this fact has been verbalized is something like, "You can't really know external reality; all you can know are the ideas in your own mind." Another way of saying it is that you, the reader, will never experience anything other than your own subjective experience. For you, you are "confined within" or "limited to" your own subjective experience.

For you, you "are" your subjective experience, nothing more. This is all you have to work with. Philosophically, if one makes the assumption that you indeed have access to "existence," but that that access is what we have just described, and you can never "know" anything beyond that subjective experience, then one is taking a "solipsistic" position. (People reject that position in various ways, of course.) And the study of that subjective experience without reference to a presumed reality "outside of it" has been termed "phenomenology."

So from your birth till your death, you will have, or deal with, or be, your own subjective experience, never anything else. In particular, you will never have someone else's subjective experience. You will never be able to experience someone else's subjective experience. Although you can imagine what you believe to be someone else's subjective experience, that imagination, what you are experiencing as you are doing that imagining, is your own subjective experience, not that of the other person. Your belief that others have subjective experience is just that, a belief, or model; it is not an observation, not experience.

Therefore, there is no way of being able to say that your subjective experience of something is the same as (or, for that matter, different from) someone else's subjective experience of that same something.

For instance, it is quite possible, and totally unascertainable one way or another, that when you see something purple, another person looking at the same thing sees it as having a color that you would call "green" if you were having that experience. "Purple" is simply what that other person has learned to call whatever color you have learned to call "green," that has a certain wave length of light. There is no way you or anyone can determine if the subjective experience of the color of that thing is the same, even though you and the other person are labeling with the same word whatever color each of you is seeing. So again, subjective experience of something by two or more people does not have to be the same, nor is it possible, or perhaps even meaningful, to say that their subjective experience is the same (or, for that matter, different).

In fact, there are actual circumstances under which it seems very likely that one person's subjective experience of something must be basically different from another's. A person with synesthesia will have an experience in two sensory modalities when others would have the experience in only one sensory modality. For that individual, specific sounds may be experienced also as specific colors. Such reported difference in experience would be hard (but probably not impossible) to explain as a difference only in labeling. But again, there is no way to ascertain, one way or another, whether your subjective experience of a particular frequency of sound waves or of light waves is the same as or different from someone else's subjective experience of that same frequency of sound or light.

So for you, your subjective experience is "everything" you experience. When you interact with someone else, and you are looking at that person, and you are empathizing with that person, all of that is "in your own mind," meaning that it is simply your subjective experience. In one sense, you are completely alone in your own universe, populated with specific subjective experiences (called "people") that are always appearing, behaving, and disappearing, in response to which you are subjectively experiencing feelings, thoughts, memories, fantasies, intentions, etc.

But that is not the whole story. None of us think that way or feel that way (at least if we are mentally healthy). We automatically make the assumption (have the belief) that we are not "alone." In fact, when we see other people and interact with them, we automatically make the assumption (have the belief) that what we are seeing is outside of us, different from us, not a part of us, but instead entities that we must interact with (and do so relatively carefully). And when we see what we call inanimate objects, we do not regard them as parts of ourselves, but instead make the assumption (have the belief) that they are parts of a larger world that we ourselves also exist in. So we feel some sort of boundary between ourselves and those other things that we do not consider to be ourselves. (This boundary has been called the "ego boundary," and its presumed existence is considered to be very important to mental health. When it is not there, or is deficient, the individual is considered to have serious illness, unless it is purposely and temporarily inhibited, as in meditative practices involving, for example, "becoming one with the universe.") But the experience of this boundary, including what you experience as being within it and outside of it, is still only your own subjective experience. (This boundary is not "outside of" your subjective experience.)

So your subjective experience is divided by you (automatically) into that which is you and that which is not, even though the boundary between what is you and what is not may be hard to define or characterize. And this creation of a boundary is an act of modeling, more specifically, the automatic making of an assumption.

(Let us note here that I have referred to this "ego boundary" phenomenon as an "assumption" and also as an "experience." An assumption is a "basic belief," a belief accepted without the necessity of meeting any legitimization criterion. It is an example of a model, since we are considering beliefs to be models. But experience itself consists of models, as discussed above. So here is an example of the lack of a clear boundary between what is "experience" and what is "beliefs about that experience." This lack of a clear boundary will be discussed further below.)

There has been speculation that the newborn infant perceives only one set of subjective experiences, a "booming, buzzing confusion," and that only with accumulation of experience and learning does the infant acquire the sense of "me" and "not me" (or "other"). To some extent this differentiation may be based upon recognizing that some subjective experience is "constant" and other subjective experience is "intermittent" (or even non-recurring), and also upon recognizing that some components of subjective experience are much more subject to control by wish or intention (i.e., likely to happen when a certain state of mind, later in the person's life to be called "intention," or "deciding," is experienced), and therefore are identified as parts of the self. For example, the reliable co-occurrence (predictability) of the experience of intending-to-move-my-hand and the experience of my-hand-moving-as-intended has led me (in infancy) to identify that intention as "my" intention and that hand as "my" hand. None of this is true of "Mother's hand."

We have been talking about some naturally occurring assumptions that we all make. Let us once again call our attention to an extremely fundamental assumption, namely, that there are others that have subjective experience just as we have. This assumption is so basic that it is even implied in this paragraph by the use of the word "we." But it is important for you to recognize that this basic assumption that you are making is just that, a basic assumption. It is not an observation. You cannot ever observe someone else's subjective experience. You just make the assumption that the other person has it. (This whole issue is very much related to the phenomenon referred to as "theory of mind.") And the nature of this basic assumption, and related issues, is part of what this presentation is about.

So we have been talking about how we naturally go beyond our solipsistic worldview, and in fact probably cannot remember ever having had it. Nevertheless, we should remember that the basic material that we have to work with is indeed our own subjective experience, nothing more. This is the "raw material" we have to work with, and it is the only raw material we will ever have to work with. The question is: "What do we do with it?"

Continuing with this "insight" (model), we can say that it is possible that many other animals have "subjective experience" just as we ourselves do. This assumption can be made because we have the impression that this subjective experience is possible by virtue of an intact and functioning brain. And since other animals have brains, we may therefore assume that they also have subjective experience. (There is no part of the human brain that is totally absent in the brains of all other animals.) We don't have to make that assumption (that other animals have subjective experience), but for our purposes, we will make that assumption and elaborate upon the idea, reserving till later the decision as to whether to retain this way of looking at things, as opposed to doing so in some other way.

So what we are saying is that all of us, including perhaps much of the animal world, have subjective experience that is dependent upon an intact and functioning brain or central nervous system. That subjective experience is "with us" (in one sense, is all that we are) from the beginning of our lives till the end. (I realize that many also believe that subjective experience persists beyond death. We will not be making that assumption, though we will also return to that question later, following better understanding of the issues involved.)

Of course, to be even more accurate, we must say that subjective experience is dependent upon an intact brain functioning in a certain way (so far not fully understood), since when the brain is asleep and not dreaming, or is anesthetized or in coma, whatever functioning is occurring is "apparently" (assumed to be) insufficient for the production of subjective experience.

(We could, of course, come up with an alternative idea, namely, that during these states the person is indeed fully aware, that is, is having unaltered subjective experience, but is unable to demonstrate it by any observable behavior, such as communication or even any kind of movement itself, only to have, also, complete amnesia for that subjective experience, produced by a temporarily induced inability to form memories. We generally do not have this belief, though the question has been raised to some extent with regard to anesthesia and vegetative states.)