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184chapter fourThe following examples will help us to understand how intrinsic andnatural the concept of upper and lower limits is for all of us in ourdaily life:1. People live in locations in this earth which are defined by an averagealtitude between two extremes. We do not live on top of thehighest mountains (the upper limit would be Mount Everest in theHimalayas) nor on land that is situated much below sea level (theJordan Valley near the Dead Sea is the lowest limit). Only a tinyminority dare to occupy the extreme regions of this earth (e.g.,mountaineers and adventurers), while the majority prefer to livein the middle between the outermost limits.2. Daylight varies throughout the year according to season and timeof the year. In Damascus, for example, the longest day of the yearis fourteen hours and twenty-six minutes, while the shortest day isonly nine hours and fifty minutes long. The other days of the yearare in between these two extremes.3. The human eye can only see colours that are visible within thespectrum of light. It cannot recognise the colours that are aboveviolet or below red.4. The human ear can only hear vibrations of sounds that are betweenthe range of twenty to twenty-thousand Hertz. Body temperature,heart rate, frequency of breathing through inhaling and exhaling,the movement of our limbs, and such—all these function naturallybetween two extremes, and if these limits were transgressed, thebody would cease to function properly.5. The amount of blood sugar in our veins varies between 70 and 120mg. Any amount in between the limits is normal, even if it is exactlyeither 70 or 120 mg, but this would trigger some tests and closersupervision. Most of us will have a healthy blood sugar levelbetween the two extremes, as we do in relation to cholesterol,blood corpuscles, fat, and other elements of the blood.6. Daytime temperatures also vary between two extremes. We may,for example, have a case where the highest temperature ever measuredin a city is 46˚C, while the lowest temperature is –5˚C. Mostdays of the year, however, do not reach these extreme temperaturesand the average is somewhere between the two. The sameapplies to the measurement of wind speed, air humidity, rainfall,or the height of waves on the sea.7. In order to prevent dehydration the human body needs a minimumamount of liquid per day, but there is no strict upper limit