Whether the final differentia is convertible with the species.


One further inquires whether the final differentia is convertible with the species.

1. And it seems not. Because the definition is said to be the boundary [terminus] of the thing. Beyond the boundary there is nothing, because nothing of the essence is beyond the definition [of the thing], but whatever belongs to its essence is within it. Since, then, several differentiae are placed in many definitions, if the final one were convertible [with the species], then the others would be superfluous or outside its essence.

2. In addition, the part is not convertible with the whole. But the differentia is part of the species, because the species is constituted from genus and differentia, and this is why, etc.

3. Moreover, each part of the definition is present plurally [ in plus], whereas the whole is present equally [ in aeque], according to the Philosopher in the second book of the Posterior Analytics But this would not be so were the final differentia convertible [with the species], and this is why, etc.

The Philosopher says the opposite.

To this, one must reply that individual and specific differentiae have to be derived from the specific forms of things, and as a result a comparison of the form itself to the form considered essentially and a comparison of the differentiae to the species considered rationally will be the same. And therefore, just as the specific form does not extend beyond the thing formednamely, that which was formed in the subordinates [ in suppositis]be-cause the form is not found beyond the thing formed nor does the thing formed exist without form (for one does not find a cow apart from the form of cow nor the form of cow apart from a cow, and the same is true for the ass and others like this). And it will be the same for a specific differentia with respect to the species, [namely] that that differentia does not go beyond the species in subordinates, nor does the species go beyond it. Nevertheless, a diffferentia is not convertible in this way with the species, because then it would include the entire understanding of the species within itself. And the differentia alone is therefore not sufficient for a definition, but rather the genus is joined to it, and to another differentia in many cases. Sometimes there are several forms in one and the same thing, or there is one form which has several operations, and then the differentia derived from one form does not include an understanding of a differentia derived from another form, just as the form does not [include the understanding of another] form. Thus the intellective [form?] does not include the understanding of the sensitive form just as a rational [body?] does not include the understanding of the differentia of what is subject to sensation.[2] And this is why if "rational substance" or "rational body" is said, it would not adequately express the understanding of a human, as it is expressed when he is called a "rational animal," because "animal" includes the understanding of the sensible.

Nevertheless, one must understand that forms of natural things are hidden from us, and this is owing to their union with matter. And this is why, when inquiring into forms, the process of natural philosophy proceeds from effects, because the sensible effects are better known to us than the forms themselves, and because the forms are hidden from us we receive few differentiae from the forms, but rather receive almost all of them from the accidents. For something rational is received from reason, which is a power of the soul, and the same is so for the sensible. And this is why in our works few differentiae are mentioned that are convertible with the species unless they are accidental differentiae, like "capable of laughter"[3] or "capable of whinnying," because the accidents are known to us. But if the differentiae were named, they would be convertible with the species, just as has been said.

1. On to the arguments. To the first, one must reply that when a later differentia does not include a prior one, then the name is not applied to the whole composed from the genus and the superior differentia, and then it is necessary to posit several differentiae in the same definition. Therefore, if the name "animal" were not applied to "animated sensible body," but only to "animated body," it would be necessary to posit in the definition of "human" both "sensible animal" and "rational animal." And this is why, because there is not one name applied to animated substance, "animated" and "sensible" are posited in the definition of animal.

2. To the second argument one must reply that something can be a part in many ways, according to the Philosopher in the fourth book of the Metaphysics., Moreover, the differentia is a part of the definition [ratio], and thus, as it is a part, so is it not convertible with the species. Therefore, it is not converted in the species according to the definition, because it is a part of the definition, but it is converted in the subordinates so that it is not found in any subordinate in which there is not a species in which the final differentia is not present, etc.

3. To the third argument one must reply that in the second book of the Posterior Analytics the Philosopher accepts general differentiae[4] and each differentia is present in ones such as these plurally [ in plus].

  • [1] "Convertible": convertibilis, i.e., interchangeable with or capable of taking the place of the species.
  • [2] The Latin is difficult here because of shifts in the gender of the adjectives employed. Unde intellectiva [feminine] non includit intellectum sensitivae, sicut nec rationale [neuter] includit intellectum differentiae huius quod est sensitivum. Our translation is one attempt, but surely not the only one possible, to make sense of the passage by anticipating the use of corpus in the next sentence.
  • [3] What distinguishes a differentia like "capable of laughter" [ risibilis] is that it is understood by most philosophers to be a proper predicate of the human, i.e., one that can only be predicated of the human being, and therefore in some sense it is convertible with the species.
  • [4] "General differentiae": differentiae communes, i.e., shared, common, or universal.
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