Abstractions are other imaginary no-things which cannot be imagined. A mathematical point has, by hypothesis, neither length, breadth nor thickness. We always think of it as a very small dot, having all three. Yet in dealing with it, mathematically, we ignore these dimensions in the result. A line has length, but no breadth or thickness. We think of it as something long and very slender. It is the edge of a solid. A plane has length and breadth but no thickness, theoretically speaking. We think of it as a very thin slice of a solid. Try to think of the surface of a solid without thinking of the solid! Abstractions are conveniences when we wish to travel light without carrying all the mystery of material reality about with us. They are like Bohr’s model atoms. And like all other unreal things, they have a danger of their own. The practical man, especially if he be a physicist or a chemist, a worker in those domains where our greatest scientific triumphs have been won, has experience always at his elbow ready to correct his errors whenever they pass beyond a certain point. Invent a machine, or a shaving soap, and you are immediately compelled to prove that your invention will work. And the only proof which those who invest in such things will accept is the sort of proof which we require of a pudding when we eat it. Experience has shown that not even the best calculations ever quite verify themselves when put to the test of novel circumstances. A small model may run smoothly, yet a large machine made in the same way will develop unexpected flaws. Even increased size is sufficient to upset the forecast. A machine may run for a week, yet become cantankerous if we try to make it run for a year. Time adds a new element. So does speed. Chemical compounds which cannot be made to explode in minute quantities--coal dust, for example--sometimes work havoc in large quantities. Now the physical sciences, like chemistry and astronomy, deal with the simplest and most tangible things of which we have any knowledge. Yet those who make them their study confess themselves baffled at every turn. In these dreadful regions, Science, the real Simon Pure, humble, beautiful and courageous, tries to maintain itself under the pitiless bombardment of facts, smoky with the infinite mystery from which they come, that rain every year more violently upon its defenseless head.