It is not illogical to think that we might fly through the air unaided, like superman, but it is impossible in principle. In order to fly we need to be lighter in effect than air, or to have the energy and surface area to create forces to counteract those of gravity, as a bird or a bat has. For this reason, to fly, we as humans need artificial aids and cannot fly like superman. This is impossible in principle, though it is not self contradictory to think of men flying like superman! Similarly, it is not illogical for grapes to grow without water on Mars, but it is impossible in principle (According to biological principles.)
Likewise, it is possible in principle to weigh someone exactly to a fraction of a gram at a given time. But if you were to ask me to tell you exactly what I weighed last week, although it is possible in principle to answer that question, it is not possible in practice, because I didn't weigh myself last week and I cannot weigh myself last week now!
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Last modified on: 27-Sep-98.
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