21. "Unobservables " is a reference similar to Immanuel Kant's distinction between phenomena ( the perceived object ). 22. Using dynamic programming to solve concrete problems is complicated by informational difficulties, such as choosing the unobservable discount rate. 23. The theory was quickly replaced by special relativity, which gave similar formulas without the existence of an unobservable aether. 24. In everyday life, there may be processes in which the increase of entropy is practically unobservable , almost zero. 25. While an experiment ensures, in confounded by unobservable factors that influenced which units received the treatment versus the control. 26. During the reign of behaviorism in the mid-20th century, unobservable phenomena such as metacognition were largely ignored. 27. Measurements c and d are redundant in the second case above, even though part of the system is unobservable . 28. One simple solution exists : if at least one of the quarks of the standard model is massless, becomes unobservable . 29. An example is Comet 11P / Tempel Swift LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. 30. The third kind is the physically unobservable , that which can never be observed by any existing sense-faculties of man.
