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It seems indeed, that the names “Genus” and “Species” are used with a number of different definitions.
I. The Conception Of Genus
First, a collection of certain things, subsisting in a certain respect with reference to one thing, and to each other, is named “genus”; according to which signification the genus of the sons of Heracles is denominated from the habitude from one thing, I mean from Hercules, and the multitude of those who derive in a certain respect alliance from him; being thus denominated, according to division from other genera.
Second, the principle of the generation of every one is again named “genus”, whether from the generator, or from the place in which some one is born. Thus, we say that Orestes derived his genus from Tantalus, and Hyllus from Heracles. And again we say, that Pindar was by genus a Theban; but Plato an Athenian: for country is a certain principle of the generation of every one, in the same manner as a father. This signification however appears to be one that may be easily adopted. For those are called sons of Heracles (“Heraclidae”) who derive their origin from the genus of Heracles; and sons of Cecrops (“Cecropidae”) who derive it from Cecrops; and also those who have an affinity to these. And the first genus is denominated that whence the principle of the generation of any one is derived; but afterwards, the multitude of those who originate from one principle, as for instance, from Hercules; which genus defining and separating from others, we call the whole collected multitude, the genus of the sons of Heracles (Heraclidae).
Third, in another way, “genus” used to name that, to which species is subjected, being thus called perhaps according to the similitude of these. For a genus of this kind is a certain principle of the things which are under it, and appears also to comprehend all the multitude which is under it.
Now that we know the name “genus” is used in three ways, the third way is that which is intended by philosophers; which also describing they explain, when they say that:
Genus is that which is predicated of many things, differing in species, in answer to the question what a thing is; as for instance, animal.
For of things which are predicated, some are predicated of one thing only, as individuals, as for instance “Socrates”, and “this man”, and “that thing”; but others are predicated of more than one thing, as Genera and Species, Differences, Peculiarities and Accidents, which are predicated of a multitude of things in common, and are not peculiar to any individual thing. But Genus is such as “animal”; and Species, such as “man”; Difference is such as “rational”; Peculiarity, such as “able to laugh”; and Accident, such as “white”, “black”, and “sitting”.
Genera therefore differ from those things which are predicated of one thing only in this–that they are predicated of a multitude of things. Genera differ from those things which are predicated of a multitude, in the following ways:
In the first place, Genus differs from Species, because though Species are predicated of a multitude, like Genus, yet they are not predicated of things differing in Species, but only in things that differ as far as they are distinct individuals (i.e., in number). Thus “man”, being a Species, is predicated of “Socrates” and “Plato” (and many other individuals of the same kind), who do not differ from each other in Species, but in number (that is, they are two different individuals). But “animal” being a Genus, is predicated of “man” and “ox”, and “horse”, which differ also in Species from each other, and not in number only.
Genus differs from Peculiarity in this, that Peculiarity is predicated of one species alone, of which it is the Peculiarity, and of the individuals under that Species. Thus “able to laugh” is predicated of only one species–“man”, and of the individuals of the belonging to species “man”; but Genus is not predicated of only one species, but of many things, which differ in Species.
Genus differs from Difference, and from common Accidents, because though Differences and Accidents which are common, are predicated of many things (like Genus), and which differ in species (like Genus), yet they are not predicated in answer to the question, “What is this?”, but in answer to the question, “What kind of a thing is this?”. For certain persons enquiring “What is this?”, of which these things are predicated, we answer with its Genus; but we do not answer with Differences and Accidents; since these are not predicated of a subject in answer to the question “What is this?”, but rather in answer to the question “What kind of a thing is this?”. For when any one asks “What kind of a thing is man?”, we say that he is “rational”; and in answer to the question what kind of a thing a “crow” is, we say that it is “black”. “Rational” however is a Difference but “black” is an Accident, But when we ask “What is man?”, we answer that “Man is an animal.”; because “animal” is the Genus of “man”.
Hence, because Genus is predicated of a multitude, it is separated from Individuals which are predicated of one thing only. But because it is predicated of things differing in Species, it is distinguished from things which are predicated as Species, or as Peculiarities. And because it is predicated in answer to the question, “What is this?”, it is separated from Differences, and from common Accidents, each of which is predicated of those things of which it is predicated, not in answer to the question “What is this?”, but in answer to the question, “What kind of a thing is this?”, or “In what manner does this subsist?”.
This descriptionof the conception of Genus is complete, as it contains nothing superfluous, nothing deficient.
II. The Conception of Species
Species, however, is predicated indeed of every form, according to which signification it is said, “Form is first worthy of imperial sway.”
That also is called Species, which is withing that conception of Genus already explained, as a division of the Genus, according to which signification we are accustomed to say that “man is a species of animal”, “animal” being a Genus; that “white is a species of colour”; and that “triangle is a species of figure”. If, however, in explaining Genus we make mention of Species, and say that “Genus is that which is predicated of many things differing in Species, in answer to the question ‘What is this?'”, and that Species is that which is within Genus; it is requisite to know that since Genus is the Genus of something and Species the Species of something, each of each, it is necessary to use both the definitions of both.
They unfold therefore the meaning of Species as follows:
Species is that which is arranged under Genus, and of which Genus is predicated in answer to the question “What is this?”.
They also explain it thus: Species is that which is predicated of many things differing in number, in answer to the question “What is this?”. This explanation, however, pertains to the most special Species, and which is Species only, but no longer Genus also; but the other explanations will pertain to Species which are not most special. What we have said however will be evident after the following explanation.
III. The Conception of the “Categories”
In each Category of things that may be predicated of subjects, there are certain things which are most general, and again others which are most special; and between things the most general and the most special there are others, which are called both Genera and Species. But the Genus which is most general, is that above which there will no longer be another superior Genus; and the most special Species is that after which there will not be another inferior Species. Between the most general Genus, and the most special Species also, there are other things which are both genera and species, when referred however to different things. But what has been said will become evident in the examination of one one the ten Categories (i.e., most general Genera). “Substance”, is indeed itself a Genus. Under this is “body”. And under “body” is “animated (body)”; under which is “animal”. Under “animal” is “rational” animal; under which is “man”. And under “man” are “Socrates”, “Plato”, and the other individuals of the human species.
Of these however “Substance” is the most general, and that which is only Genus; and “man” is most special, and that which is only Species. But “body” is a Species of Substance, and the Genus of “animated (body)”. “Animated (body)” is both a Species of “body”, and the Genus of “animal”. Again, “animal” is a Species indeed of “animated (body)”, but the Genus of “rational (animal)”. And “rational (animal)”, is a Species indeed of “animal”, but the Genus of “man”. And “man” is a Species of “rational (animal)”, but is not the Genus anything, but is Species alone. Everything prior to individuals, which is proximately predicated of them, will be Species only, and not Genus also. Hence as “Substance”, which is in the highest place, is most general (because there is no Genus prior to it); thus also “man” being a Species, after which there is no other Species, nor any thing which is capable of being divided into Species, but Individuals will be Species alone, and the last species, and as we have said, the most special species. But the thing in between the most general Genus and the most special Species, called the “media”, will be the Species of the things prior to them; and the Genera of things posterior to them.
Hence these “media” have two relations, one to things prior to them, according to which they are said to be the Species of them, but the other, to things posterior to them, according to which they are said to be the Genera of them. But the most general Genus and most special Species, called the “extremes” have only one relation. For that which is most general, has indeed a relation to the things which are under it, since it is the highest Genus of all things; but has no relation to things prior to it, being supreme, and the first principle, and, as we have said, that above which there will not be another superior Genus. The most special species also has one relation, as towards things prior to it, of which it is the Species; yet it has not a different relation, as towards things posterior to it; but it is said to be the Species of individuals, as comprehending them, and again, the Species of things prior to it, as being comprehended by them. The most general Genus therefore is defined to be that which being Genus is not Species. And again, it is that above which there will not be another superior Genus. But the most special Species, is defined to be that, which being Species is not Genus; and that which being Species we cannot divide into Species. Farther still, it is that also which is predicated of many things differing in number, in answer to the question “What is this?”.
But the “media” of the “extremes”, are called “subaltern species and genera”, and each of them is admitted to be both Genus and Species, with reference however to different things. For the things prior to the most special Species, in an ascent as far as to the most general Genus, are called “subaltern genera and species”. Thus Agamemnon is a son of Atreus, a son of Pelops, a son of Tantalus, and in the last place, of Jupiter. In genealogies however, they refer, for the most part, to one principle, for instance to Jupiter; but in Genera and Species this is not the case; for “being” is not the common Genus of all things, nor, as Aristotle says, are all things “homogenous”, according to one supreme Genus. But the first ten Genera are arranged, as in the Categories, as the first ten principles. And though some one should call all things “beings”, yet, says he, he will call them so homonymously, and not synonymously. For if “being” were the common Genus of all things, all things would be synonymously denominated “beings”. But the first principles being ten, the communion in being called “beings” is in the name only, and not also in the definition pertaining to the name. The most general Genera therefore are ten; but the most special Species are indeed contained in a certain number, yet not in an infinite number. Individuals however, which are after the most special Species are infinite.
Hence, when we have descended as far as to the most special Species from the most general Genera, Plato orders us to rest; but advises us to descend through those things which are in the middle, dividing by specific Differences. But infinites, says he, are to be dismissed; for of these there cannot be any scientific knowledge (“science”). In descending therefore to the most special Species, it is necessary to proceed by dividing the multitude; but in ascending to the most general Genera, it is necessary to collect divisions of the multitude into one. For Species is collective of the multitude into one nature, and Genus possesses this power in a still greater degree. On the contrary, things which subsist according to a part, and particulars, always divide the one into multitude. For that which is particular has always a divisive power; but that which is common has the power of collecting and uniting.
With respect to Genus and Species therefore, having explained what each of them is, and since Genus is one thing, but Species a number of things, (for the division of Genus is always into more than one Species) Genus indeed is always predicated of Species, and all prior things are predicated of posterior things; but Species is neither predicated of the Genus prior to it, nor of the Genera prior to that; for does Species does not reciprocate with its Genus. For it is necessary, that things can be predicated of “coordinate” things, as “able to neigh” is predicated of “horse”; and superior things can be predicated of inferior things, as “animal” of “man”; but inferior things cannot be predicated of superior things. You can not say that “animal” is “man”, as you can say that “man” is an “animal”.
But of those things of which Species is predicated, of those, the Genus of the Species is also necessarily predicates, and the Genus of that Genus, as far as to the most general Genus. For, if it is true to say that Socrates is “man”, and that “man” is “animal”, and that “animal” is “substance”: it is also true to say that Socrates is “animal” and “substance”. For since superiors are always predicated of inferiors, Species indeed is predicated of an Individual; but Genus is predicated both of Species and an Individuals; and the most general Genus is predicated of Genus, or Genera (if the media and subalterns are many), and also of Species, and an Individual. For the most general Genus is predicated of all the Genera, Species, and Individuals contained under it; but the Genus which is prior to the most special Species is predicated of all the most special Species and Individuals. And that which is Species alone is predicated of all the Individuals of that Species; but an Individual is predicated of one particular thing alone. An Individual, however, is such as “Socrates”, “this white substance”, and “this man who approaches”. But things of this kind are called Individuals, because each of them consists of Peculiarities, of which the collection can never belong to any other thing. For the same Peculiarities can be predicated of Socrates, cannot be predicated of any other person. The same Peculiarities however of “man”, I mean of man considered as a class, can be inherent in many, or rather in all individual men, so far as they are “man”. Hence the individual is contained by Species, and Species by Genus. For Genus is a certain whole; but the individual is a part; and Species is both whole (in relation to the individuals it collects) and part (in relation to the Genus which collects it and others). It is a part of something else, but not a whole of anything else, but subsists in other things; for the whole is in its parts.
Concerning Genus and Species therefore, we have shown what they are, and also what that which is most general, and that which is most special are, what things are both Genera and Species, what are Individuals, and in how many ways Genus and Species are assumed.
Classical Reasoning I, Assessment