Whilst in Botany and Zoology the species is fixed at the lowest step of the classification (varieties not being reckoned as classes), and the genus is also fixed on the step next above it, in Logic these predicables are treated as movable up and down the ladder: any lower class being species in relation to any higher; which higher class, wherever taken, thus becomes a genus. Lion may logically be regarded as a species of digitigrade, or mammal, or animal; and then each of these is a genus as to lion: or, again, digitigrade may be regarded as a species of mammal, or mammal as a species of animal. The highest class, however, is never a species; wherefore it is called a _Summum Genus_: and the lowest class is never a genus; wherefore it is called an _Infima Species_. Between these two any step may be either species or genus, according to the relation in which it is viewed to other classes, and is then called Subaltern. The _summum genus_, again, may be viewed in relation to a _given_ universe or _suppositio_ (that is, any limited area of existence now the object of attention), or to the _whole_ universe. If we take the animal kingdom as our _suppositio_, Animal is the _summum genus_; but if we take the whole universe, 'All things' is the _summum genus_. "Porphyry's tree" is used to illustrate this doctrine. It begins with a _summum genus_, 'Substance,' and descends by adding differences, step by step, to the _infima species_, 'Man.' It also illustrates Division by Dichotomy. SUBSTANCE / \ CORPOREAL || INCORPOREAL || BODY / \ ANIMATE || INANIMATE || LIVING BODY / \ SENSIBLE || INSENSIBLE || ANIMAL / \ RATIONAL || IRRATIONAL || MAN //||\\ // || \\ // || \\ // || \\ // || \\ // || \\ // || \\ _Socrates_ _Plato_ _Aristotle_ Beginning with 'Substance,' as _summum genus_, and adding the difference 'Corporeal,' we frame the species 'Body.' Taking 'Body' as the genus and adding the difference 'Animate,' we frame the species 'Living Body;' and so on till 'Man' is reached; which, being _infima species_, is only subdivisible into individuals. But the division of Man into individuals involves a change of principle; it is a division of the denotation, not an increase of the connotation as in the earlier steps. Only one side of each dichotomy is followed out in the 'tree': if the other side had been taken, Incorporeal Substance would be 'Spirit'; which might be similarly subdivided.