Night and Day
How firm is the distinction between night and day? What is night and what is day? What kind of distinction is this? Clearly there are periods during which it is neither night nor day—at dusk and dawn. Night is turning into day, but it is neither one nor the other. We don’t have a simple name for this time period, which lasts I would say for about 20 minutes; we could call it “nay” or “dight”. So the distinction between night and day is not sharp and not exhaustive. Nor is it universal: there is no night on the sun, and no day at the center of the earth. Things that are always bathed in light have no night, and things that are always dark have no day. In the land of the blind the distinction presumably makes no sense, since light is not sensed there; the distinction is not apparent to blind people, except perhaps by testimony. Is what we would call night really day for nocturnal creatures with superior vision to ours? They can see at night as well as we can see during the day, and they conduct their waking activities in our night; perhaps sunlight blinds them so that everything is dark for them during our day.  Do they invert our day and night? What if the moon was larger and always reflected a lot of sunlight onto the earth—would that abolish night for us? The sleeping hours might be just like an overcast day. But then, is it really night for us now when the moon is bright and full? What about eclipses—do they literally turn day into night? And what if heat replaced light as the stimulus for vision—wouldn’t that undermine the night-day distinction? If things were warmer at night than during the day, people would see better at night, so there might be a linguistic switch with the words “night” and “day”. The distinction between day and night is friable and relative, pragmatic and contextual; there is no rigid absolute dichotomy. I can imagine a principled eliminativism about night and day: there are really no such things, not as aspects of objective reality; there is just a continuously varying amount of light and grades of visual acuity. You don’t find physicists talking much about the physics of day and night; the whole idea is centered on human concerns and powers. We can certainly imagine beings that have no use for the distinction, but without missing out on any objective matter of fact. When the Beatles sang about a Hard Day’s Night they were reflecting on the superficiality of the distinction. If night is defined as the hours of sleep, its status as objective looks distinctly questionable; it is perfectly possible to do a day’s work in the night hours (so called). People often say that two things differ as night and day, as if this was a rigid and sharp distinction; but in fact the distinction is vague, pragmatic, and relative.
Note that in another use of “day” we include the night as part of the day. You can say with perfect propriety, “What day is it today?” in the dead of night. The names of days of the week clearly include the period we call night. These distinctions map onto objective motions of the planet and are not as soft and pliable as the use of “day” in contrast to “night”. The semantics of these words ties them to certain anthropocentric conditions: “x is day” is true if and only if x has enough light to make human activity feasible, or the human eye can make things out clearly. We then reify this notion into something that seems to us to transcend human concerns, but reflection suggests that night and day are not independent of our particular contingent place in nature (they are part of the “relative conception” not the “absolute conception”). Just imagine if we became nocturnal because of some mutation in our eyes: we sleep when the sun casts the most light, are able to see only when the light is low, and are most active between the hours of 10pm and 6am; then we would surely start to refer to the hours of the 24-hour day quite differently. Night is when we can’t see so well, tend to feel sleepy, and want to stay home; day is when we see clearly, are wide awake, and feel like venturing forth. We do better, conceptually, to think of night as just another part of the day—the part in which the day is darker, quieter, and scarier. Alternatively, day is just the attenuation of night, the time when night becomes less opaque. If the difference of sunlight were less dramatic, with a lighter night and a gloomier day, we would surely think this way. This is one of those cases in which ordinary language (and ordinary thought) mislead us about reality: the words and their use make us think that we are dealing with a more solid and durable distinction than we really are. Rectifying this tendency, we could find ourselves more willing to stay at home during the day and more willing to go out at night (the “late day”); and be less prone to feeling guilty about afternoon naps and middle-of-the-night snacks. We need to free ourselves from the tyranny of the night-day dichotomy. The sun and the moon have vied for human worship; it is time to grant them equal status. Sun-time and moon-time are all parts of the same continuous day. The distinction between day and night is not written into the fabric of the universe. The day is bright night and the night is dark day.
 Is it analytic that the night is dark? How dark? What if we had artificial light everywhere? The starry sky is quite bright at night. Is it analytic that the day is light? How light? What if the earth were enveloped in a light-blocking mist? The cloudy sky can be quite dark at noon. So it is unclear that lightness and darkness can be used to define day and night more precisely.