A System of Logic: Ratiocinative and Inductive Volume I Part 7

The various kinds of definition which these distinctions give rise to, and the purposes to which they are respectively subservient, will be minutely considered in the proper place.

3. According to the above view of essential propositions, no proposition can be reckoned such which relates to an individual by name, that is, in which the subject is a proper name. Individuals have no essences. When the schoolmen talked of the essence of an individual, they did not mean the properties implied in its name, for the names of individuals imply no properties. They regarded as of the essence of an individual, whatever was of the essence of the species in which they were accustomed to place that individual; _i.e._ of the cla.s.s to which it was most familiarly referred, and to which, therefore, they conceived that it by nature belonged. Thus, because the proposition Man is a rational being, was an essential proposition, they affirmed the same thing of the proposition, Julius Csar is a rational being. This followed very naturally if genera and species were to be considered as ent.i.ties, distinct from, but _inhering_ in, the individuals composing them. If _man_ was a substance inhering in each individual man, the _essence_ of man (whatever that might mean) was naturally supposed to accompany it; to inhere in John Thompson, and to form the _common essence_ of Thompson and Julius Csar. It might then be fairly said, that rationality, being of the essence of Man, was of the essence also of Thompson. But if Man altogether be only the individual men and a name bestowed upon them in consequence of certain common properties, what becomes of John Thompson's essence?

A fundamental error is seldom expelled from philosophy by a single victory. It retreats slowly, defends every inch of ground, and often, after it has been driven from the open country, retains a footing in some remote fastness. The essences of individuals were an unmeaning figment arising from a misapprehension of the essences of cla.s.ses, yet even Locke, when he extirpated the parent error, could not shake himself free from that which was its fruit. He distinguished two sorts of essences, Real and Nominal. His nominal essences were the essences of cla.s.ses, explained nearly as we have now explained them. Nor is anything wanting to render the third book of Locke's Essay a nearly unexceptionable treatise on the connotation of names, except to free its language from the a.s.sumption of what are called Abstract Ideas, which unfortunately is involved in the phraseology, though not necessarily connected with the thoughts contained in that immortal Third Book.[24]

But, besides nominal essences, he admitted real essences, or essences of individual objects, which he supposed to be the causes of the sensible properties of those objects. We know not (said he) what these are; (and this acknowledgment rendered the fiction comparatively innocuous;) but if we did, we could, from them alone, demonstrate the sensible properties of the object, as the properties of the triangle are demonstrated from the definition of the triangle. I shall have occasion to revert to this theory in treating of Demonstration, and of the conditions under which one property of a thing admits of being demonstrated from another property. It is enough here to remark that, according to this definition, the real essence of an object has, in the progress of physics, come to be conceived as nearly equivalent, in the case of bodies, to their corpuscular structure: what it is now supposed to mean in the case of any other ent.i.ties, I would not take upon myself to define.

4. An essential proposition, then, is one which is purely verbal; which a.s.serts of a thing under a particular name, only what is a.s.serted of it in the fact of calling it by that name; and which therefore either gives no information, or gives it respecting the name, not the thing.

Non-essential, or accidental propositions, on the contrary, may be called Real Propositions, in opposition to Verbal. They predicate of a thing some fact not involved in the signification of the name by which the proposition speaks of it; some attribute not connoted by that name.

Such are all propositions concerning things individually designated, and all general or particular propositions in which the predicate connotes any attribute not connoted by the subject. All these, if true, add to our knowledge: they convey information, not already involved in the names employed. When I am told that all, or even that some objects, which have certain qualities, or which stand in certain relations, have also certain other qualities, or stand in certain other relations, I learn from this proposition a new fact; a fact not included in my knowledge of the meaning of the words, nor even of the existence of Things answering to the signification of those words. It is this cla.s.s of propositions only which are in themselves instructive, or from which any instructive propositions can be inferred.[25]

Nothing has probably contributed more to the opinion so long prevalent of the futility of the school logic, than the circ.u.mstance that almost all the examples used in the common school books to ill.u.s.trate the doctrine of predication and that of the syllogism, consist of essential propositions. They were usually taken either from the branches or from the main trunk of the Predicamental Tree, which included nothing but what was of the _essence_ of the species: _Omne corpus est substantia_, _Omne animal est corpus_, _Omnis h.o.m.o est corpus_, _Omnis h.o.m.o est animal_, _Omnis h.o.m.o est rationalis_, and so forth. It is far from wonderful that the syllogistic art should have been thought to be of no use in a.s.sisting correct reasoning, when almost the only propositions which, in the hands of its professed teachers, it was employed to prove, were such as every one a.s.sented to without proof the moment he comprehended the meaning of the words; and stood exactly on a level, in point of evidence, with the premises from which they were drawn. I have, therefore, throughout this work, avoided the employment of essential propositions as examples, except where the nature of the principle to be ill.u.s.trated specifically required them.

5. With respect to propositions which do convey information--which a.s.sert something of a Thing, under a name that does not already presuppose what is about to be a.s.serted; there are two different aspects in which these, or rather such of them as are general propositions, may be considered: we may either look at them as portions of speculative truth, or as memoranda for practical use. According as we consider propositions in one or the other of these lights, their import may be conveniently expressed in one or in the other of two formulas.

According to the formula which we have hitherto employed, and which is best adapted to express the import of the proposition as a portion of our theoretical knowledge, All men are mortal, means that the attributes of man are always accompanied by the attribute mortality: No men are G.o.ds, means that the attributes of man are never accompanied by the attributes, or at least never by all the attributes, signified by the word G.o.d. But when the proposition is considered as a memorandum for practical use, we shall find a different mode of expressing the same meaning better adapted to indicate the office which the proposition performs. The practical use of a proposition is, to apprise or remind us what we have to expect, in any individual case which comes within the a.s.sertion contained in the proposition. In reference to this purpose, the proposition, All men are mortal, means that the attributes of man are _evidence of_, are a _mark_ of, mortality; an indication by which the presence of that attribute is made manifest. No men are G.o.ds, means that the attributes of man are a mark or evidence that some or all of the attributes understood to belong to a G.o.d are not there; that where the former are, we need not expect to find the latter.

These two forms of expression are at bottom equivalent; but the one points the attention more directly to what a proposition means, the latter to the manner in which it is to be used.

Now it is to be observed that Reasoning (the subject to which we are next to proceed) is a process into which propositions enter not as ultimate results, but as means to the establishment of other propositions. We may expect, therefore, that the mode of exhibiting the import of a general proposition which shows it in its application to practical use, will best express the function which propositions perform in Reasoning. And accordingly, in the theory of Reasoning, the mode of viewing the subject which considers a Proposition as a.s.serting that one fact or phenomenon is a _mark_ or _evidence_ of another fact or phenomenon, will be found almost indispensable. For the purposes of that Theory, the best mode of defining the import of a proposition is not the mode which shows most clearly what it is in itself, but that which most distinctly suggests the manner in which it may be made available for advancing from it to other propositions.

CHAPTER VII.

OF THE NATURE OF CLa.s.sIFICATION, AND THE FIVE PREDICABLES.

1. In examining into the nature of general propositions, we have adverted much less than is usual with logicians to the ideas of a Cla.s.s, and Cla.s.sification; ideas which, since the Realist doctrine of General Substances went out of vogue, have formed the basis of almost every attempt at a philosophical theory of general terms and general propositions. We have considered general names as having a meaning, quite independently of their being the names of cla.s.ses. That circ.u.mstance is in truth accidental, it being wholly immaterial to the signification of the name whether there are many objects, or only one, to which it happens to be applicable, or whether there be any at all.

G.o.d is as much a general term to the Christian or Jew as to the Polytheist; and dragon, hippogriff, chimera, mermaid, ghost, are as much so as if real objects existed, corresponding to those names. Every name the signification of which is const.i.tuted by attributes, is potentially a name of an indefinite number of objects; but it needs not be actually the name of any; and if of any, it may be the name of only one. As soon as we employ a name to connote attributes, the things, be they more or fewer, which happen to possess those attributes, are const.i.tuted _ipso facto_ a cla.s.s. But in predicating the name we predicate only the attributes; and the fact of belonging to a cla.s.s does not, in many cases, come into view at all.

Although, however, Predication does not presuppose Cla.s.sification, and though the theory of Names and of Propositions is not cleared up, but only enc.u.mbered, by intruding the idea of cla.s.sification into it, there is nevertheless a close connexion between Cla.s.sification and the employment of General Names. By every general name which we introduce, we create a cla.s.s, if there be any things, real or imaginary, to compose it; that is, any Things corresponding to the signification of the name.

Cla.s.ses, therefore, mostly owe their existence to general language. But general language, also, though that is not the most common case, sometimes owes its existence to cla.s.ses. A general, which is as much as to say a significant, name, is indeed mostly introduced because we have a signification to express by it; because we need a word by means of which to predicate the attributes which it connotes. But it is also true that a name is sometimes introduced because we have found it convenient to create a cla.s.s; because we have thought it useful for the regulation of our mental operations, that a certain group of objects should be thought of together. A naturalist, for purposes connected with his particular science, sees reason to distribute the animal or vegetable creation into certain groups rather than into any others, and he requires a name to bind, as it were, each of his groups together. It must not however be supposed that such names, when introduced, differ in any respect, as to their mode of signification, from other connotative names. The cla.s.ses which they denote are, as much as any other cla.s.ses, const.i.tuted by certain common attributes, and their names are significant of those attributes, and of nothing else. The names of Cuvier's cla.s.ses and orders, _Plantigrades_, _Digitigrades_, &c., are as much the expression of attributes as if those names had preceded, instead of grown out of, his cla.s.sification of animals. The only peculiarity of the case is, that the convenience of cla.s.sification was here the primary motive for introducing the names; while in other cases the name is introduced as a means of predication, and the formation of a cla.s.s denoted by it is only an indirect consequence.

The principles which ought to regulate Cla.s.sification as a logical process subservient to the investigation of truth, cannot be discussed to any purpose until a much later stage of our inquiry. But, of Cla.s.sification as resulting from, and implied in, the fact of employing general language, we cannot forbear to treat here, without leaving the theory of general names and of their employment in predication, mutilated and formless.

2. This portion of the theory of general language is the subject of what is termed the doctrine of the Predicables; a set of distinctions handed down from Aristotle, and his follower Porphyry, many of which have taken a firm root in scientific, and some of them even in popular, phraseology. The predicables are a five-fold division of General Names, not grounded as usual on a difference in their meaning, that is, in the attribute which they connote, but on a difference in the kind of cla.s.s which they denote. We may predicate of a thing five different varieties of cla.s.s-name:--

A _genus_ of the thing (_?????_).

A _species_ (_e?d??_).

A _differentia_ (_d?af???_).

A _proprium_ (_?d???_).

An _accidens_ (_s?e????_).

It is to be remarked of these distinctions, that they express, not what the predicate is in its own meaning, but what relation it bears to the subject of which it happens on the particular occasion to be predicated.

There are not some names which are exclusively genera, and others which are exclusively species, or differenti; but the same name is referred to one or another predicable, according to the subject of which it is predicated on the particular occasion. _Animal_, for instance, is a genus with respect to man, or John; a species with respect to Substance, or Being. _Rectangular_ is one of the Differenti of a geometrical square; it is merely one of the Accidentia of the table at which I am writing. The words genus, species, &c. are therefore relative terms; they are names applied to certain predicates, to express the relation between them and some given subject: a relation grounded, as we shall see, not on what the predicate connotes, but on the cla.s.s which it denotes, and on the place which, in some given cla.s.sification, that cla.s.s occupies relatively to the particular subject.

3. Of these five names, two, Genus and Species, are not only used by naturalists in a technical acceptation not precisely agreeing with their philosophical meaning, but have also acquired a popular acceptation, much more general than either. In this popular sense any two cla.s.ses, one of which includes the whole of the other and more, may be called a Genus and a Species. Such, for instance, are Animal and Man; Man and Mathematician. Animal is a Genus; Man and Brute are its two species; or we may divide it into a greater number of species, as man, horse, dog, &c. _Biped_, or _two-footed animal_, may also be considered a genus, of which man and bird are two species. _Taste_ is a genus, of which sweet taste, sour taste, salt taste, &c. are species. _Virtue_ is a genus; justice, prudence, courage, fort.i.tude, generosity, &c. are its species.

The same cla.s.s which is a genus with reference to the sub-cla.s.ses or species included in it, may be itself a species with reference to a more comprehensive, or, as it is often called, a superior genus. Man is a species with reference to animal, but a genus with reference to the species Mathematician. Animal is a genus, divided into two species, man and brute; but animal is also a species, which, with another species, vegetable, makes up the genus, organized being. Biped is a genus with reference to man and bird, but a species with respect to the superior genus, animal. Taste is a genus divided into species, but also a species of the genus sensation. Virtue, a genus with reference to justice, temperance, &c., is one of the species of the genus, mental quality.

In this popular sense the words Genus and Species have pa.s.sed into common discourse. And it should be observed that in ordinary parlance, not the name of the cla.s.s, but the cla.s.s itself, is said to be the genus or species; not, of course, the cla.s.s in the sense of each individual of the cla.s.s, but the individuals collectively, considered as an aggregate whole; the name by which the cla.s.s is designated being then called not the genus or species, but the generic or specific name. And this is an admissible form of expression; nor is it of any importance which of the two modes of speaking we adopt, provided the rest of our language is consistent with it; but, if we call the cla.s.s itself the genus, we must not talk of predicating the genus. We predicate of man the _name_ mortal; and by predicating the name, we may be said, in an intelligible sense, to predicate what the name expresses, the _attribute_ mortality; but in no allowable sense of the word predication do we predicate of man the _cla.s.s_ mortal. We predicate of him the fact of belonging to the cla.s.s.

By the Aristotelian logicians, the terms genus and species were used in a more restricted sense. They did not admit every cla.s.s which could be divided into other cla.s.ses to be a genus, or every cla.s.s which could be included in a larger cla.s.s to be a species. Animal was by them considered a genus; man and brute co-ordinate species under that genus: _biped_, however, would not have been admitted to be a genus with reference to man, but a _proprium_ or _accidens_ only. It was requisite, according to their theory, that genus and species should be of the _essence_ of the subject. Animal was of the essence of man; biped was not. And in every cla.s.sification they considered some one cla.s.s as the lowest or _infima_ species. Man, for instance, was a lowest species. Any further divisions into which the cla.s.s might be capable of being broken down, as man into white, black, and red man, or into priest and layman, they did not admit to be species.

It has been seen, however, in the preceding chapter, that the distinction between the essence of a cla.s.s, and the attributes or properties which are not of its essence--a distinction which has given occasion to so much abstruse speculation, and to which so mysterious a character was formerly, and by many writers is still, attached,--amounts to nothing more than the difference between those attributes of the cla.s.s which are, and those which are not, involved in the signification of the cla.s.s-name. As applied to individuals, the word Essence, we found, has no meaning, except in connexion with the exploded tenets of the Realists; and what the schoolmen chose to call the essence of an individual, was simply the essence of the cla.s.s to which that individual was most familiarly referred.

Is there no difference, then, save this merely verbal one, between the cla.s.ses which the schoolmen admitted to be genera or species, and those to which they refused the t.i.tle? Is it an error to regard some of the differences which exist among objects as differences _in kind_ (_genere_ or _specie_), and others only as differences in the accidents? Were the schoolmen right or wrong in giving to some of the cla.s.ses into which things may be divided, the name of _kinds_, and considering others as secondary divisions, grounded on differences of a comparatively superficial nature? Examination will show that the Aristotelians did mean something by this distinction, and something important; but which, being but indistinctly conceived, was inadequately expressed by the phraseology of essences, and the various other modes of speech to which they had recourse.

4. It is a fundamental principle in logic, that the power of framing cla.s.ses is unlimited, as long as there is any (even the smallest) difference to found a distinction upon. Take any attribute whatever, and if some things have it, and others have not, we may ground on the attribute a division of all things into two cla.s.ses; and we actually do so, the moment we create a name which connotes the attribute. The number of possible cla.s.ses, therefore, is boundless; and there are as many actual cla.s.ses (either of real or of imaginary things) as there are general names, positive and negative together.

But if we contemplate any one of the cla.s.ses so formed, such as the cla.s.s animal or plant, or the cla.s.s sulphur or phosphorus, or the cla.s.s white or red, and consider in what particulars the individuals included in the cla.s.s differ from those which do not come within it, we find a very remarkable diversity in this respect between some cla.s.ses and others. There are some cla.s.ses, the things contained in which differ from other things only in certain particulars which may be numbered, while others differ in more than can be numbered, more even than we need ever expect to know. Some cla.s.ses have little or nothing in common to characterize them by, except precisely what is connoted by the name: white things, for example, are not distinguished by any common properties, except whiteness; or if they are, it is only by such as are in some way dependent on, or connected with, whiteness. But a hundred generations have not exhausted the common properties of animals or of plants, of sulphur or of phosphorus; nor do we suppose them to be exhaustible, but proceed to new observations and experiments, in the full confidence of discovering new properties which were by no means implied in those we previously knew. While, if any one were to propose for investigation the common properties of all things which are of the same colour, the same shape, or the same specific gravity, the absurdity would be palpable. We have no ground to believe that any such common properties exist, except such as may be shown to be involved in the supposition itself, or to be derivable from it by some law of causation.

It appears, therefore, that the properties, on which we ground our cla.s.ses, sometimes exhaust all that the cla.s.s has in common, or contain it all by some mode of implication; but in other instances we make a selection of a few properties from among not only a greater number, but a number inexhaustible by us, and to which as we know no bounds, they may, so far as we are concerned, be regarded as infinite.

There is no impropriety in saying that, of these two cla.s.sifications, the one answers to a much more radical distinction in the things themselves, than the other does. And if any one even chooses to say that the one cla.s.sification is made by nature, the other by us for our convenience, he will be right; provided he means no more than this: Where a certain apparent difference between things (though perhaps in itself of little moment) answers to we know not what number of other differences, pervading not only their known properties, but properties yet undiscovered, it is not optional but imperative to recognise this difference as the foundation of a specific distinction; while, on the contrary, differences that are merely finite and determinate, like those designated by the words white, black, or red, may be disregarded if the purpose for which the cla.s.sification is made does not require attention to those particular properties. The differences, however, are made by nature, in both cases; while the recognition of those differences as grounds of cla.s.sification and of naming, is, equally in both cases, the act of man: only in the one case, the ends of language and of cla.s.sification would be subverted if no notice were taken of the difference, while in the other case, the necessity of taking notice of it depends on the importance or unimportance of the particular qualities in which the difference happens to consist.

Now, these cla.s.ses, distinguished by unknown mult.i.tudes of properties, and not solely by a few determinate ones--which are parted off from one another by an unfathomable chasm, instead of a mere ordinary ditch with a visible bottom--are the only cla.s.ses which, by the Aristotelian logicians, were considered as genera or species. Differences which extended only to a certain property or properties, and there terminated, they considered as differences only in the _accidents_ of things; but where any cla.s.s differed from other things by an infinite series of differences, known and unknown, they considered the distinction as one of _kind_, and spoke of it as being an _essential_ difference, which is also one of the current meanings of that vague expression at the present day.

Conceiving the schoolmen to have been justified in drawing a broad line of separation between these two kinds of cla.s.ses and of cla.s.s-distinctions, I shall not only retain the division itself, but continue to express it in their language. According to that language, the proximate (or lowest) Kind to which any individual is referrible, is called its species. Conformably to this, Sir Isaac Newton would be said to be of the species man. There are indeed numerous sub-cla.s.ses included in the cla.s.s man, to which Newton also belongs; for example, Christian, and Englishman, and Mathematician. But these, though distinct cla.s.ses, are not, in our sense of the term, distinct Kinds of men. A Christian, for example, differs from other human beings; but he differs only in the attribute which the word expresses, namely, belief in Christianity, and whatever else that implies, either as involved in the fact itself, or connected with it through some law of cause and effect. We should never think of inquiring what properties, unconnected with Christianity either as cause or effect, are common to all Christians and peculiar to them; while in regard to all Men, physiologists are perpetually carrying on such an inquiry; nor is the answer ever likely to be completed. Man, therefore, we may call a species; Christian, or Mathematician, we cannot.

Note here, that it is by no means intended to imply that there may not be different Kinds, or logical species, of man. The various races and temperaments, the two s.e.xes, and even the various ages, may be differences of kind, within our meaning of the term. I do not say that they are so. For in the progress of physiology it may almost be said to be made out, that the differences which really exist between different races, s.e.xes, &c., follow as consequences, under laws of nature, from a small number of primary differences which can be precisely determined, and which, as the phrase is, _account for_ all the rest. If this be so, these are not distinctions in kind; no more than Christian, Jew, Mussulman, and Pagan, a difference which also carries many consequences along with it. And in this way cla.s.ses are often mistaken for real Kinds, which are afterwards proved not to be so. But if it turned out that the differences were not capable of being thus accounted for, then Caucasian, Mongolian, Negro, &c. would be really different Kinds of human beings, and ent.i.tled to be ranked as species by the logician; though not by the naturalist. For (as already noticed) the word species is used in a different signification in logic and in natural history. By the naturalist, organized beings are not usually said to be of different species, if it is supposed that they could possibly have descended from the same stock. That, however, is a sense artificially given to the word, for the technical purposes of a particular science. To the logician, if a negro and a white man differ in the same manner (however less in degree) as a horse and a camel do, that is, if their differences are inexhaustible, and not referrible to any common cause, they are different species, whether they are descended from common ancestors or not. But if their differences can all be traced to climate and habits, or to some one or a few special differences in structure, they are not, in the logician's view, specially distinct.

When the _infima species_, or proximate Kind, to which an individual belongs, has been ascertained, the properties common to that Kind include necessarily the whole of the common properties of every other real Kind to which the individual can be referrible. Let the individual, for example, be Socrates, and the proximate Kind, man. Animal, or living creature, is also a real Kind, and includes Socrates; but, since it likewise includes man, or in other words, since all men are animals, the properties common to animals form a portion of the common properties of the sub-cla.s.s, man. And if there be any cla.s.s which includes Socrates without including man, that cla.s.s is not a real Kind. Let the cla.s.s for example, be _flat-nosed_; that being a cla.s.s which includes Socrates, without including all men. To determine whether it is a real Kind, we must ask ourselves this question: Have all flat-nosed animals, in addition to whatever is implied in their flat noses, any common properties, other than those which are common to all animals whatever?

If they had; if a flat nose were a mark or index to an indefinite number of other peculiarities, not deducible from the former by an ascertainable law, then out of the cla.s.s man we might cut another cla.s.s, flat-nosed man, which according to our definition, would be a Kind. But if we could do this, man would not be, as it was a.s.sumed to be, the proximate Kind. Therefore, the properties of the proximate Kind do comprehend those (whether known or unknown) of all other Kinds to which the individual belongs; which was the point we undertook to prove. And hence, every other Kind which is predicable of the individual, will be to the proximate Kind in the relation of a genus, according to even the popular acceptation of the terms genus and species; that is, it will be a larger cla.s.s, including it and more.

We are now able to fix the logical meaning of these terms. Every cla.s.s which is a real Kind, that is, which is distinguished from all other cla.s.ses by an indeterminate mult.i.tude of properties not derivable from one another, is either a genus or a species. A Kind which is not divisible into other Kinds, cannot be a genus, because it has no species under it; but it is itself a species, both with reference to the individuals below and to the genera above (Species Prdicabilis and Species Subjicibilis.) But every Kind which admits of division into real Kinds (as animal into mammal, bird, fish, &c., or bird into various species of birds) is a genus to all below it, a species to all genera in which it is itself included. And here we may close this part of the discussion, and pa.s.s to the three remaining predicables, Differentia, Proprium, and Accidens.

5. To begin with Differentia. This word is correlative with the words genus and species, and as all admit, it signifies the attribute which distinguishes a given species from every other species of the same genus. This is so far clear: but we may still ask, which of the distinguis.h.i.+ng attributes it signifies. For we have seen that every Kind (and a species must be a Kind) is distinguished from other Kinds not by any one attribute, but by an indefinite number. Man, for instance, is a species of the genus animal: Rational (or rationality, for it is of no consequence here whether we use the concrete or the abstract form) is generally a.s.signed by logicians as the Differentia; and doubtless this attribute serves the purpose of distinction: but it has also been remarked of man, that he is a cooking animal; the only animal that dresses its food. This, therefore, is another of the attributes by which the species man is distinguished from other species of the same genus: would this attribute serve equally well for a differentia? The Aristotelians say No; having laid it down that the differentia must, like the genus and species, be of the _essence_ of the subject.

And here we lose even that vestige of a meaning grounded in the nature of the things themselves, which may be supposed to be attached to the word essence when it is said that genus and species must be of the essence of the thing. There can be no doubt that when the schoolmen talked of the essences of things as opposed to their accidents, they had confusedly in view the distinction between differences of kind, and the differences which are not of kind; they meant to intimate that genera and species must be Kinds. Their notion of the essence of a thing was a vague notion of a something which makes it what it is, _i. e._ which makes it the Kind of thing that it is--which causes it to have all that variety of properties which distinguish its Kind. But when the matter came to be looked at more closely, n.o.body could discover what caused the thing to have all those properties, nor even that there was anything which caused it to have them. Logicians, however, not liking to admit this, and being unable to detect what made the thing to be what it was, satisfied themselves with what made it to be what it was called. Of the innumerable properties, known and unknown, that are common to the cla.s.s man, a portion only, and of course a very small portion, are connoted by its name; these few, however, will naturally have been thus distinguished from the rest either for their greater obviousness, or for greater supposed importance. These properties, then, which were connoted by the name, logicians seized upon, and called them the essence of the species; and not stopping there, they affirmed them, in the case of the _infima species_, to be the essence of the individual too; for it was their maxim, that the species contained the "whole essence" of the thing. Metaphysics, that fertile field of delusion propagated by language, does not afford a more signal instance of such delusion. On this account it was that rationality, being connoted by the name man, was allowed to be a differentia of the cla.s.s; but the peculiarity of cooking their food, not being connoted, was relegated to the cla.s.s of accidental properties.

The distinction, therefore, between Differentia, Proprium, and Accidens, is not grounded in the nature of things, but in the connotation of names; and we must seek it there, if we wish to find what it is.

From the fact that the genus includes the species, in other words _de_notes more than the species, or is predicable of a greater number of individuals, it follows that the species must connote more than the genus. It must connote all the attributes which the genus connotes, or there would be nothing to prevent it from denoting individuals not included in the genus. And it must connote something besides, otherwise it would include the whole genus. Animal denotes all the individuals denoted by man, and many more. Man, therefore, must connote all that animal connotes, otherwise there might be men who are not animals; and it must connote something more than animal connotes, otherwise all animals would be men. This surplus of connotation--this which the species connotes over and above the connotation of the genus--is the Differentia, or specific difference; or, to state the same proposition in other words, the Differentia is that which must be added to the connotation of the genus, to complete the connotation of the species.

The word man, for instance, exclusively of what it connotes in common with animal, also connotes rationality, and at least some approximation to that external form which we all know, but which as we have no name for it considered in itself, we are content to call the human. The Differentia, or specific difference, therefore, of man, as referred to the genus animal, is that outward form and the possession of reason. The Aristotelians said, the possession of reason, without the outward form.

But if they adhered to this, they would have been obliged to call the Houyhnhnms men. The question never arose, and they were never called upon to decide how such a case would have affected their notion of essentiality. However this may be, they were satisfied with taking such a portion of the differentia as sufficed to distinguish the species from all other _existing_ things, though by so doing they might not exhaust the connotation of the name.

6. And here, to prevent the notion of differentia from being restricted within too narrow limits, it is necessary to remark, that a species, even as referred to the same genus, will not always have the same differentia, but a different one, according to the principle and purpose which preside over the particular cla.s.sification. For example, a naturalist surveys the various kinds of animals, and looks out for the cla.s.sification of them most in accordance with the order in which, for zoological purposes, he considers it desirable that we should think of them. With this view he finds it advisable that one of his fundamental divisions should be into warm-blooded and cold-blooded animals; or into animals which breathe with lungs and those which breathe with gills; or into carnivorous, and frugivorous or graminivorous; or into those which walk on the flat part and those which walk on the extremity of the foot, a distinction on which two of Cuvier's families are founded. In doing this, the naturalist creates as many new cla.s.ses; which are by no means those to which the individual animal is familiarly and spontaneously referred; nor should we ever think of a.s.signing to them so prominent a position in our arrangement of the animal kingdom, unless for a preconceived purpose of scientific convenience. And to the liberty of doing this there is no limit. In the examples we have given, most of the cla.s.ses are real Kinds, since each of the peculiarities is an index to a mult.i.tude of properties belonging to the cla.s.s which it characterizes: but even if the case were otherwise--if the other properties of those cla.s.ses could all be derived, by any process known to us, from the one peculiarity on which the cla.s.s is founded--even then, if these derivative properties were of primary importance for the purposes of the naturalist, he would be warranted in founding his primary divisions on them.

If, however, practical convenience is a sufficient warrant for making the main demarcations in our arrangement of objects run in lines not coinciding with any distinction of Kind, and so creating genera and species in the popular sense which are not genera or species in the rigorous sense at all, _ fortiori_ must we be warranted, when our genera and species _are_ real genera and species, in marking the distinction between them by those of their properties which considerations of practical convenience most strongly recommend. If we cut a species out of a given genus--the species man, for instance, out of the genus animal--with an intention on our part that the peculiarity by which we are to be guided in the application of the name man should be rationality, then rationality is the differentia of the species man.