Every Genus is a Species of the class next higher than itself; and every Species is a Genus of the classes lower than itself. Thus it would seem that the extension in either direction would be infinite. But, for the purposes of finite thought, the authorities teach that there must be a Highest Genus, which cannot be the Species of a higher class, and which is called the _Summum Genus_. The _Summum Genus_ is expressed by terms such as the following: "Being;" "Existence;" "The Absolute;" "Something;" "Thing;" "The Ultimate Reality," or some similar term denoting the state of being _ultimate_. Likewise, at the lowest end of the scale we find what are called the Lowest Species, or _Infima Species_. The Infima Species are always _individuals_. Thus we have the _individual_ at one end of the scale; and _The Absolute_ at the other. Beyond these limits the mind of man cannot travel. There has been much confusion in making classifications and some ingenious plans have been evolved for simplifying the process. That of Jevons is perhaps the simplest, when understood. This authority says: "All these difficulties are avoided in the _perfect logical method of dividing each Genus into two Species, and not more than two, so that one species possesses a particular quality, and the other does not_. Thus if I divide dwelling-houses into those which are made of brick and those which are not made of brick, I am perfectly safe and nobody can find fault with me.... Suppose, for instance, that I divide dwelling-houses as below: Dwelling-House | --+------+-------+-------+-------+-- | | | | | Brick Stone Earth Iron Wood "The evident objection will at once be made, that houses may be built of other materials than those here specified. In Australia, houses are sometimes made of the bark of gum-trees; the Esquimaux live in snow houses; tents may be considered as canvas houses, and it is easy to conceive of houses made of terra-cotta, paper, straw, etc. All logical difficulties will, however, be avoided _if I never make more than two species at each step_, in the following way:-- Dwelling-House | +----+----+ | | Brick Not-Brick | +----+----+ | | Stone Not-Stone | +----+----+ | | Wooden Not-Wooden | +----+----+ | | Iron Not-Iron "It is quite certain that I must in this division have left a place for every possible kind of house; for if a house is not made of brick, nor stone, nor wood, nor iron, it yet comes under the species at the right hand, which is not-iron, not-wooden, not-stone, and not-brick.... This manner of classifying things may seem to be inconvenient, but it is in reality the only logical way."