The only universal principles that need be considered here are the Law of Causation and the Form of Substance and Attribute (Mill’s doctrine of Natural Kinds): which may be called the principles of parallel reasoning; because the greater part of ordinary reasoning consists in drawing some inference parallel to one or the other of them, usually in some restricted shape. For example, a restriction of causation is the proposition that “exposure to intense daylight causes sun-burn,” of substance and attribute that “the specific gravity of gold is about 19·5”: whence we infer that if we expose ourselves to sunlight our faces or hands will suffer, or that any piece of gold will be relatively very heavy. But in such cases erroneous inferences are easy: for example, to expect that exposure to London sunshine will cause sunburn; for there the foul atmosphere cuts off the actinic rays: or to expect that a lump of brass will have specific gravity 19·5. A necessary precaution before trusting an inference, therefore, is the ascertaining that the inference deals with the very same sort of case as the premise describes: else there is no complete parallel. And Logicians show in the form of the syllogism this necessary precaution—to use their favourite example in a case of Substance and Attribute: Major premise—_All men are mortal_; Minor premise—_Socrates is a man_; ⁂ Conclusion—_Socrates is mortal_. The conclusion _Socrates is Mortal_ is parallel to the major premise _All men are mortal_; and that it deals with the very same sort of case is secured by the minor premise, _Socrates is a man_. Whoever reasons must see to it that this premise is true. But the savage has never noticed that necessity; and thence come most of his errors.