Close your eyes and try to imagine how the world looks when no one is looking. You can’t, of course, because the “look” of something necessarily involves someone looking. This conundrum has bedeviled philosophers for centuries. Some people think we create our own reality. Others think our experience is dictated by atoms and genes. But our experience depends on both factors together—the inner and the outer. It’s always an interaction of self and world.

If the grass looks green in the springtime, there must be something about it different from brown grass at the end of summer or the red skin of a ripe cherry. That something is chlorophyll. But why does chlorophyll produce in us the sensation of greenness rather than of the redness associated with the cherry or the brownness of dry grass? This sort of question involves the perceiver’s active role in perception and the evaluative process at the base of consciousness. It calls to mind the adage about the grass on the other side of the fence. Perhaps greenness signifies something to the motivated perceiver, in the way that pain signifies tissue damage and hunger signifies a need for nourishment. But what?

This is a question that falls through the gap between science and philosophy. Science has defined itself as a study of the natural world, not of the perceiver’s subjective experience. Consciousness has long been an embarrassment for the physical sciences. Even psychology often embraces this “objective” approach; for decades it was more about the behavior of rats than the subjective experience of people. Philosophy, on the other hand, is willing to ask apparently silly questions, like: “Could I experience as red what you experience as green?” The answer, I hold, is no: to the degree we are physically similar, you and I can expect to have a similar experience of chlorophyll-bearing leaves. Because we are variations of the same creature, we evaluate perceptual input similarly.

I think of conscious experience as a conversation the brain has with itself. Words have meanings by social convention. These arise through common interaction with the world to which they largely refer. Perhaps sensations get their characteristic quality in a similar way—as “words” in the language of the senses. The quality of greenness is a convention (not social but genetic) arising through evolutionary interaction with the world over thousands of generations. In other words, sensations must have an evolutionary history, just as words have an etymological history. The “meaning” of a perceptual quality such as greenness refers to its role within the brain/body’s internal communications, just as a given word plays a specific role within human communication. We have, of course, a grand ability to play with words. To a limited extent, we are able to play with perception too. Though I cannot will to see green as red, yet imagination, hallucination, drugs, painting and stained glass all give me access to greenness as a pure experience disengaged from the things usually associated with it. Experiments demonstrate that human perception can indeed adapt to color filters that switch red for green input to the eyes. With time, the grass eventually returns to looking green the way it normally does.

What the world looks like “when no one is looking” can only mean what it looks like to the organism one happens to be. There is no “real” way things look apart from someone looking, just as there is no intrinsic meaning to words apart from language users. Naturally, it is functional for some organisms to distinguish wavelengths of light. However, the behavioral ability to distinguish red from green does not answer the question of why chlorophyll looks green rather than red, let alone why the world looks like anything at all. Yet, it is a clue. For, one’s conscious perceptual experience is organized in a particular and largely consistent way that reflects one’s needs as a living organism and as a member of the human species in particular.

Stability and structure in the world is largely matched by the stability and structure of the organism’s perception. Organisms must evaluate stimuli in order to respond in a way that allows them to survive at least long enough to reproduce. Perception is about evaluation, which is expressed in the way things look. For example, pain, fear and hunger evaluate the significance of a stimulus in terms of the body’s needs. The behavioral “meaning” of pain could be compared to the verbal warning: watch out! Whatever the meaning of greenness, it is not as evident as that of bodily sensations, nor does color bear their urgency. Color does have associations and can bear emotional charge. However, like hearing—and unlike touch—vision is a distance sense. Because of distance from the stimulus, there is time to monitor events and evaluate them in relative safety and detachment. Because of their negligible momentum, the physical impact of photons on the retina is nil. In contrast, pain occurs when the body has already been damaged through direct contact.

Phenomenal qualities in general are like the intelligible meanings that emerge through the babble of spoken syllables or the symbols on a written page. The greenness of green, like the hurtfulness of pain, contains a “self-evident” meaning that arises in much the way that meaning in language does. Just as squiggles on a page come alive as a story, “qualia” are how the brain/body represents and interprets to itself the internal connections it has itself made to bear significance for it. I call this assertive process fiat—a command as in “Let there be light!” Or a supposition as in “Let x stand for such-and-such.” This is the fundamental conjuring act that calls forth the world into consciousness. It is the action of an agent rather than the reaction of a passive causal system. In this case, the brain/body decrees with royal authority: “Let there be green, to represent grass.”

This brings to mind another “silly” question of philosophy: could a person behave exactly as they do, in every detail, but without conscious awareness? Again I say no. The fact that we are not “dark” inside is functional; conscious experience is essential to human survival. There must be some way that the world looks and sounds and smells and feels if one is to respond to it in all the sophisticated ways that human creatures do. There must be a story we narrate to ourselves not only in words, but directly in the language of the senses. Given that one is conscious at all, and can distinguish certain wavelengths, the grass looks green first of all because it must look somehow. Secondly, it looks green because that experience is the tag assigned to a particular input, just as one has learned to tag that input with the word ‘green’. The word may seem inseparable from its meaning, just as greenness seems inseparable from a certain wavelength of light, but both are conventions asserted by fiat.

What can we learn from the philosophy of perception? First and foremost: that our experience is not an open window directly revealing the world as it “really” is; but neither is it a private illusion. Instead, perception is an interaction between a living organism, with its needs and drives, and the external world (whatever that turns out to be). Consciousness is produced by the brain/body in response to physical stimuli. It is a co-creation of mind and world. Understanding that enables and requires us to take responsibility for how we interpret our experience as well as how we behave in response. That’s a daunting challenge, to be sure, since two factors operating jointly always result in ambiguity and uncertainty. Yet, one cannot afford to ignore either factor for the sake of certainty or convenience. What appears self-evident or objectively true is always open to interpretation. Appearances cannot be taken at face value. We must always question our assumptions and the motivations that underlie them.