necessity for taking precautions against a danger depends not so much on its probability or improbability, as on the degree of its probability; and to an equal degree on the greatness of the danger that impends.  If the occurrence of a small mishap has a probability say of even 75 per cent, there may be little necessity of guarding against it; while if the danger of total destruction has a probability as low as even 1 per cent, we should guard against it sedulously.

The more complicated the question, the more elements involved, the more difficult it is to settle it wisely by mere discussion.  The effort of the imagination of each person must be directed not so much to getting a correct mental picture of what the words employed describe, as to getting a correct picture of what the person using the words desires them to describe.  Any person who has had experience in discussions of this character knows what an effort this is, even if he is talking with persons whom he has known for years, and with whose mental and lingual characteristics he is well acquainted:  and he also knows how much more difficult it is when he is talking with persons whom he knows but slightly.

It may here be pointed out how greatly the imaginations of men differ, and how little account is taken of this difference in every-day life.  In poetry and fiction imagination is recognized; and it is also recognized to some extent in painting, inventing, and, in general, in “the arts.”  But in ordinary life, the difference among men in imagination is almost never noticed.  Yet a French proverb is “point d’imagination, point de grand general”; and Napoleon indicated a danger from untrained imagination in his celebrated warning to his generals not to make “pictures” to themselves of difficulties and disasters.

The difference in imagination among men is shown clearly by the difference—­and often the differences—­between inventors and engineers, and the scarcity of men who are both inventors and engineers.  Ericsson repudiated the suggestion that he was an inventor, and stoutly and always declared he was an engineer.  This was at a time, not very long ago, when it was hardly respectable to be an inventor; when, even though men admitted that some inventors had done valuable work, the work was supposed to be largely a chance shot of a more or less crazy man.  Yet Ericsson was an inventor—­though he was an engineer.  So were Sir William Thompson (afterward Lord Kelvin), Helmholtz, Westinghouse, and a very few others; so are Edison and Sperry.  Many inventors, however, live in their imaginations mainly—­some almost wholly.  Like Pegasus, they do not like to be fastened to a plough or anything else material.  Facts, figures, and blue-prints fill their souls with loathing, and bright generalities delight them.  The engineer, on the other hand, is a man of brass and iron and logarithms; in imagination he is blind, in flexibility he resembles reinforced concrete.  He is the antipodes of the inventor; he despises the inventor, and the inventor hates him.  Fortunately, however, there is a little bit of the inventor in most engineers, and a trace of the engineer in most inventors; while in some inventors there is a good deal of the engineer.  And once in a while we meet a man who carries both natures in his brain.  That man does marvels.