Sometimes this fallacy is caused by assuming that because a certain result followed an event in the only case known, therefore there was a causal connection.  In a hearing before a committee of the Massachusetts legislature on a bill to establish closer relations between Boston and its suburbs, the question was asked of a witness whether he believed that in the case of London “the London police would have been as efficient as they are now if there had been no annexation” of the surrounding towns; he very properly replied:  “That’s a hard question to answer, because we have only the existing side to look at.  We don’t know what it would have been as separate communities.”  Wherever multiple causes are possible for a phenomenon it is unsafe to argue from a single case.

Another form of error in reasoning to a cause is to assume that a fact is simple, when it is really complex, as in the following example: 

I do not think I am overstepping the bounds when I say that the headship of no corporation, or state, or even the headship of the United States, requires greater general ability, force of character, or knowledge of administration than the head of administration of a great city like New York or Berlin.  The latter we know to be well administered, the former—­well, let us say, less so.  The whole difference is in the systems.  Apply the Berlin system to New York, and you will get Berlin results.

Here the writer wholly ignores all sorts of active causes for this difference:  Berlin has a tolerably homogeneous population, New York the most heterogeneous in the world; Germans by nature respect law and authority, and hanker for centralization; Americans make and break laws light-heartedly, and are restive under authority; and one might easily go further.

Arguments that national prosperity has followed a higher or a lower tariff are especially apt to be vitiated by this error.  It is not that the tariff has no relation to the prosperity, but that there are other causes intermingled with it which may have had more immediate effect.  A bad grain crop or a season of reckless speculation may obliterate all the traceable causes of a change in the tariff.  Arguments from motive, too, are apt to fall into this error.  It is notorious that human motives are mixed.  If you argue that a whole class of business organizations are evil because they have been formed solely for the purpose of making inordinate and oppressive profits, you leave out of sight a motive which is strong among American business men—­the interest in seeing a great business more efficiently managed, and the desire to exercise power beneficently; and your argument suffers from its illegitimate assumption of a simple cause.  So in the same way if you are arguing for or against the advantages of the elective system in a school or a college, or of a classical education, or of athletics, it would be folly to assume that any one cause or effect covered the whole case.  Whenever in an argument you are trying to establish any such large and complex fact, you must be wary lest you thus assume a single cause where in reality there are a legion of causes.