The name in Latin, post hoc ergo propter hoc, means “after this therefore because of this”. The fallacy is committed when it is assumed that because Y follows X that Y was caused by X. Also known as coincidental correlation.
- "A remarkable meteor was seen in the sky, and followed by a dreadful national calamity: a conjuction among the planets was followed by a royal marriage which issued in far-reaching consequences; and the superstitious conclude that one of the facts had some kind of causal connection with the other. We have outlived these weaknesses of past ages: but we have not outgrown the fallacies on which they proceeded. A country or college has prospered under a certain government or management, and some conclude that it was because of the government or management and oppose all projected improvements." (McCosh, The Laws of Discursive Thought, pp. 193-4.)
- "For his 72nd birthday Wally received a cellular phone, and after three years of using it almost daily, he was diagnosed with brain cancer. Clearly, holding the phone so close to his head was responsible." (Carrick & Finsen, The Persuasive Pen, p. 196.)
- "The standard example of the fallacy is the old Kentish peasant's argument that Tenterden Steeple was the cause of Goodwin Sands. Sir Thomas More ... had been sent down into Kent as a commissioner to inquire into the cause of the silting up of Sandwich Haven. Among those who came to his court was the oldest inhabitant, and thinking that he from his great age must at least have seen more than anybody else, More asked him what he had to say as to the cause of the sands. 'Forsooth, sir,' was the greybeard's answer, 'I am an old man: I think that Tenterden Steeple is the cause of Goodwin Sands. For I am an old man, and I may remember the building of Tenterden Steeple, and I may remember when there was no steeple at all there. And before that Tenterden Steeple was in building, there was no manner of speaking of any flats or sands that stopped the haven; and, therefore, I think that Tenterden Steeple is the cause of the destroying and decaying of Sandwich Haven.'" (Minto, Logic: Inductive and Deductive, pp. 295-6.)
- The most vulgar form of this fallacy is that which is commonly called post hoc ergo propter hoc or cum hoc ergo propter hoc. As when it was inferred that England owed her industrial preeminence to her restrictions on commerce; as when the old school of financiers and some speculative writers maintained that the national debt was one of the causes of national prosperity; as when the excellence of the Church, of the Houses of Lords and Commons, of the procedure of the law courts, &c., were inferred from the mere fact that the country had prospered under them. In such cases as these, if it can be rendered probable by other evidence that the supposed causes have some tendency to produce the effect ascribed to them, the fact of its having been produced though only in one instance is of some value as a verification by specific experience; but in itself it goes scarcely any way at all towards establishing such a tendency since admitting the effect, a hundred other antecedents could show an equally strong title of that kind to be considered as the cause. (Mill, A System of Logic, 1900, p. 519.)
- "A man indulges freely in wine at his club and wakes up the next morning with a splitting headache, and he attributes his indisposition to the intense mental application with which he has been prosecuting his business. The clouds are dispersed, and this is ascribed to the rising of the full moon. A man is irritable in the morning, and his relatives account for it by the fact that he got up on the left side of the bed." (Toohey, An Elementary Handbook of Logic, p. 191.)
- An example is provided by a female passenger on board the Italian liner Andrea Doria. On the fatal night of Doria's collision with the Swedish ship Gripsholm, off Nantucket in 1956, the lady retired to her cabin and flicked a light switch. Suddenly there was a great crash, and grinding metal, and passengers and crew ran screaming through the passageways. The lady burst from her cabin and explained to the first person in sight that she must have set the ship's emergency brake! (Fischer, Historians' Fallacies, p. 166.)
It has been argued, in the same manner, that when two things are constantly observed to accompany one another, the one must depend upon the other. Thence, for ex. in natural science, classes have been formed from two properties which have been found to exist together in certain animals or plants; and it has been asserted that one must depend upon the other, and that therefore every object possessed of one must have both. Some time after, a new being has been discovered possessed of one of these properties and not of the other, and the whole theory falls to the ground. Nothing has contributed more to the overthrow of physiological systems, than the discovery of so many anomalous animals and plants in New Holland. (Bentham, Outline of a New System of Logic, pp. 254-5.)