"On the invariability of law depend the security of scientific experiment, and all power of planning a result and of predicting the future. On this the chemist rests, sure that Nature will ever respond in the same way, if he be precise in putting his questions. A variation in his results is taken by him as implying a change in his procedure, not a change in Nature. And so with all human action; the more it is based on knowledge, the more secure is it in its forecastings, for all “accident" is the result of ignorance, and is due to the working of laws whose presence was unknown or overlooked. In the mental and moral worlds, as much as in the physical, results can be foreseen, planned for, calculated on. Nature never betrays us; we are betrayed by our own blindness. p. 7"