In cases where _post hoc_ is mistaken for _propter hoc_, simple sequence for causal sequence, there is commonly some bias of prejudice or custom which fixes observation on some one antecedent and diverts attention from other circumstances and from what may be observed to follow in other cases. In the minds of Digby and his followers there was probably a veneration for the sword as the weapon of honour, and a superstitious belief in some secret sympathy between the sword and its owner. So when the practice of poisoning was common, and suspicion was flurried by panic fear, observation was often at fault. Pope Clement VIII. was said to have been killed by the fumes of a poisoned candle which was placed in his bedroom. Undoubtedly candles were there, but those who attributed the Pope's death to them took no notice of the fact that a brazier of burning charcoal was at the same time in the apartment with no sufficient outlet for its fumes. Prince Eugene is said to have received a poisoned letter, which he suspected and immediately threw from him. To ascertain whether his suspicions were well founded the letter was administered to a dog, which, to make assurance doubly sure, was fortified by an antidote. The dog died, but no inquiry seems to have been made into the character of the antidote. Hotspur's retort to Glendower showed a sound sense of the true value to be attached to mere priority. _Glendower_. At my nativity The front of heaven was full of fiery shapes, Of burning cressets: and at my birth The frame and huge foundation of the earth Shaked like a coward. _Hotspur_. Why so it would have done at the same season, if your mother's cat had but kittened, though yourself had never been born. 1 Hen. IV., 3, 1, 13. We all admit at once that the retort was just. What principle of sound conclusion was involved in it? It is the business of Inductive Logic to make such principles explicit. Taking _Post Hoc ergo Propter Hoc_ as a generic name for fallacious arguments of causation based on observed facts, for the fallacious proof of causation from experience, the question for Logic is, What more than mere _sequence_ is required to prove _consequence?_ When do observations of _Post Hoc_ warrant the conclusion _Propter Hoc?_ II.--MEANING OF "CAUSE".--METHODS OF OBSERVATION--MILL'S EXPERIMENTAL METHODS.