The conditions surrounding John and Thomas are those of the simplest kind, and the conversation between them of the most uncomplicated character.

But when--not two people but--say a dozen or more, are considering highly complicated questions, such as the House Naval Committee discuss when officers are called to testify before them, no two of the twenty congressmen can form the same mental picture when an officer uses the word--say "fleet." The reason is simply that very few of the congressmen hearing that word have ever seen a fleet; none of them know exactly what it is, and every one forms a picture which is partly the result of all his previous education and experience; which are different from the previous education and experience of every other congressman on the committee.

Furthermore, no one of the officers uses words exactly as the other officers do; and the English language is too vague (or rather the usual interpretation put on words is too vague) to assure us that even ordinary words are mutually understood.

For instance, the question is asked: "Do you consider it probable that such or such a thing would happen ?" Now what does the questioner mean by "probable," and what does the officer think he means?
Mathematically, the meaning of "probable" is that there is more than 50 per cent of chance that the thing would happen; but who in ordinary conversation uses that word in that way?
That this is not an academic point is shown by the fact that if the answer is "no" the usual inference from the answer is that there is no need for guarding against the contingency.

Yet such an inference, if the word "probable" were used correctly by both the questioner and the answerer, would be utterly unjustified, because the 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.