On Being and Essence

The name “essence”, then, is not taken from being said in the second way; for some things are called beings in this way that have no essence, as is clear in privations. Rather, essence is taken from being said in the first way. Hence the Commentator says in the same place: “Being said in the first way is what signifies the substance of a thing.” And since, as has been said, being said in this way is divided into the ten genera, essence must signify something common to all the natures through which different beings are placed in different genera and different species; as humanity is the essence of man, and so of the others.note

And since that through which a thing is constituted in its proper genus or species is what we signify by the definition indicating what the thing is, it comes about that philosophers change the name “essence” into the name “quiddity”. And this is what the Philosopher, in Book 7 of the Metaphysics, often calls “what it was to be”, that is, that through which something has existence as a what. It is also called form, inasmuch as form signifies the perfection or definiteness of each thing, as Avicenna says in Book 2 of his Metaphysics.note

This is also called by another name, “nature”, taking nature in the first of the four senses that Boethius assigns in On the Two Natures: namely, the sense in which nature is said to be whatever can in any way be grasped by the intellect. For a thing is intelligible only through its definition and essence. And so the Philosopher too says in Book 4 of the Metaphysics that every substance is a nature. But the name “nature” taken in this way seems to signify the essence of a thing insofar as it has an order or ordination to the thing’s proper operation, since no thing lacks its proper operation. The name “quiddity”, for its part, is taken from the fact that it is signified by the definition; but it is called “essence” inasmuch as through it and in it a thing has existence.note

That matter alone is not the essence is plain. For a thing is knowable through its essence, and through it is placed in a species or genus; but matter is not a principle of knowledge, nor is anything determined to a species or genus according to it, but only according to that by which something is in act. Nor again can form alone be called the essence of a composite substance, although some try to assert this. For from what has been said it is clear that essence is what is signified by the definition of a thing; and the definition of natural substances contains not only form but also matter. Otherwise natural definitions and mathematical ones would not differ.

Nor can it be said that matter is placed in the definition of a natural substance as something added to its essence, or as a being outside its nature or essence. For this mode belongs properly to accidents, which do not have an essence perfectly; hence they must include in their definition the substance or subject, which is outside their genus. It is clear, therefore, that essence comprises matter and form. Nor can it be said that essence signifies the relation between matter and form, or something added over and above them. For this would necessarily be an accident extraneous to the thing, and the thing would not be known through it—all of which belong to essence.

For through form, which is the act of matter, matter becomes a being in act and a this-something. Hence what supervenes does not give matter existence in act without qualification, but existence in act of such a kind, as accidents do: whiteness, for example, makes something actually white. Hence when such a form is acquired, the thing is not said to be generated without qualification, but in a qualified sense.

Hence the essence by which a thing is called a being must be neither the form alone nor the matter alone, but both, although form alone is in its own way the cause of this existence, or essence. We see this in other things constituted from several principles: a thing is not named from one of those principles alone, but from what embraces both. This is clear in flavors. Sweetness is caused by the action of heat digesting moisture; and although in this way heat is the cause of sweetness, a body is not called sweet from its heat, but from the flavor that embraces both the hot and the moist. But since matter is the principle of individuation, it might perhaps seem to follow from this that essence, which embraces in itself form as well as matter, is only particular and not universal.

For if animal were not the whole of what man is, but a part of him, it would not be predicated of him, since no integral part is predicated of its whole. How this comes about can be seen if we examine body as it is posited as a part of animal and as it is posited as a genus; for it cannot be a genus in the same way in which it is an integral part. This name “body”, then, can be taken in several ways. Body, as it is in the category of substance, is so called from having such a nature that three dimensions can be designated in it; whereas the three designated dimensions themselves are the body that is in the genus of quantity. Now it happens in things that what has one perfection attains a further perfection as well, as is clear in man, who has a sensitive nature and, beyond that, an intellective one.

Likewise, over and above this perfection, which is to have such a form that three dimensions can be designated in it, another perfection can be added, such as life or something of the kind. So this name “body” can designate a certain thing that has such a form that from it follows the designability of three dimensions in it, by way of exclusion: that is, such that no further perfection follows from that form, but if anything else is added, it is outside the meaning of “body” so understood. In this way body will be a material and integral part of animal, because in this way the soul will be outside what is signified by the name “body”, and will supervene upon the body itself, so that from these two, namely from soul and body, as from parts, animal is constituted.note

This name “body” can also be taken so as to signify a certain thing that has such a form that three dimensions can be designated in it, whatever that form may be, whether some further perfection can come from it or not. And in this way body will be the genus of animal, because there is nothing to be found in animal that is not implicitly contained in body.

For the soul is not a form other than that through which three dimensions could be designated in that thing. And so, when I said that body is what has such a form that three dimensions can be designated in it, I meant whatever that form might be, whether soul, or stoneness, or any other form. In this way the form of animal is implicitly contained in body, or in the form of body, inasmuch as body is its genus. And such too is the relation of animal to man.

For if “animal” named only a thing that has a certain perfection such that it can sense and be moved by a principle existing within it, by way of exclusion of any other perfection, then whatever other perfection supervened would stand to animal as a part, and not as implicitly contained in the notion of animal; and animal would not be a genus. But it is a genus inasmuch as it signifies a certain thing from whose form sensation and motion can come, whatever that form may be, whether a sensitive soul only, or a soul at once sensitive and rational.

Thus, then, the genus signifies indeterminately the whole of what is in the species (for it does not signify matter alone). Likewise the difference too signifies the whole of what is in the species, and not form alone; and the definition also signifies the whole, as does the species—but in different ways. For the genus signifies the whole as a certain denomination that determines what is material in the thing, without determination of its proper form; hence the genus is taken from matter, although it is not matter. Hence it is clear that “body” means what has such a perfection that three dimensions can be designated in it; and this perfection stands, as something material, to a further perfection.

The difference, conversely, is like a certain determination taken from a determinate form, without determinate matter belonging to its primary understanding. This is clear when we say “animate”, that is, what has a soul: it is not determined what it is, whether a body or something else. Hence Avicenna says that the genus is not in the difference as a part of its essence, but only as a being outside its quiddity or essence, just as the subject belongs to the understanding of its attributes. And so the genus is not predicated of the difference, speaking per se, as Aristotle says in Book 3 of On the Soul and Book 4 of the Topics, except perhaps as a subject is predicated of an attribute. But the definition, or the species, comprises both: namely the determinate matter that the name of the genus designates, and the determinate form that the name of the difference designates.

For the concept of animal expresses the nature of a thing without determination of the specific form, from what is material with respect to the ultimate perfection. The concept of this difference, “rational”, consists in the determination of the specific form. From these two concepts the concept of the species, or of the definition, is constituted. And so, just as a thing constituted from certain things does not receive the predication of the things from which it is constituted, so neither does a concept receive the predication of the concepts from which it is constituted: for we do not say that the definition is the genus or the difference.

And so the Commentator says, in Book 2 of the Metaphysics, that prime matter is called one through the removal of all forms, whereas the genus is called one through the commonness of the designated form. Hence it is clear that when the difference is added, and that indetermination which was the cause of the unity of the genus is removed, species remain that are different in essence. And since, as has been said, the nature of the species is indeterminate with respect to the individual, as the nature of the genus is with respect to the species, it follows that, just as what is the genus, as it is predicated of the species, implies in its meaning, though indistinctly, the whole of what is determinately in the species, so what is the species, as it is predicated of the individual, must signify the whole of what is essentially in the individual, though indistinctly. It is in this way that the essence of the species is signified by the name “man”; hence “man” is predicated of Socrates.

But if the nature of the species is signified by way of exclusion of designated matter, which is the principle of individuation, then it will stand as a part. And this is how the name “humanity” signifies: for humanity signifies that by which a man is a man. But designated matter is not that by which a man is a man; and so it is in no way contained among the things from which a man has it that he is a man.

Since, then, humanity includes in its concept only those things from which a man has it that he is a man, it is clear that determinate or designated matter is excluded or cut off from its meaning. And since a part is not predicated of the whole, it follows that humanity is predicated neither of man nor of Socrates. Hence Avicenna says that the quiddity of a composite is not the composite itself whose quiddity it is, even though the quiddity itself is composite. Thus humanity, though composite, is not man; on the contrary, it must be received in something that is designated matter.

But since, as has been said, the designation of the species with respect to the genus is through forms, while the designation of the individual with respect to the species is through matter, it follows that the name signifying that from which the nature of the genus is taken, by way of exclusion of the determinate form that perfects the species, must signify the material part of the whole, as body is the material part of man. The name signifying that from which the nature of the species is taken, by way of exclusion of designated matter, signifies the formal part. And so humanity is signified as a certain form, and is said to be the form of the whole. This is not as though it were added over and above the essential parts, matter and form, as the form of a house is added over and above its integral parts. Rather, it is the form that is the whole, that is, embracing form and matter, yet by way of exclusion of those things through which matter is apt to be designated.

Thus it is clear that the essence of man is signified by the name “man” and by the name “humanity”, but in different ways, as has been said. The name “man” signifies it as a whole, inasmuch as it does not express the designation of matter but contains it implicitly and indistinctly, as we said that the genus contains the difference; and so the name “man” is predicated of individuals. But the name “humanity” signifies it as a part, and contains in its meaning only what belongs to man insofar as he is man, and cuts off all designation of matter; hence it would not be predicated of individual men. And for this reason the name “essence” is sometimes found predicated of a thing—for Socrates is said to be a certain essence—and sometimes denied of it, as when we say that the essence of Socrates is not Socrates.

Likewise, it cannot be said that the notion of genus, species and difference belongs to essence insofar as it is a certain thing existing outside singulars, as the Platonists held; for then genus and species would not be predicated of this individual. For it cannot be said that Socrates is what is separate from him; nor would that separate thing contribute to the knowledge of this designated singular. And so it remains that the notion of genus, species or difference belongs to essence insofar as it signifies as a whole, as in the name “man” or “animal”, inasmuch as it contains implicitly and indistinctly the whole of what is in the individual.

Now nature or essence taken in this way can be considered in two ways. In one way, according to its proper nature and notion; and this is its absolute consideration. In this way nothing is true of it except what belongs to it as such; hence whatever else is attributed to it, the attribution is false. For example, rational and animal and the other things that fall within his definition belong to man insofar as he is man; but white or black, or anything of this kind that is not part of the notion of humanity, does not belong to man insofar as he is man. So if it is asked whether this nature can be called one or many, neither should be granted, because both are outside the concept of humanity, and either can happen to it.

For if plurality were part of its notion, it could never be one, although it is one as it is in Socrates. Likewise, if unity were part of its concept and notion, then the nature of Socrates and of Plato would be one and the same, and it could not be multiplied in many. In the second way it is considered according to the existence it has in this or that individual; and in this way something is predicated of it accidentally, by reason of that in which it is, as it is said that man is white because Socrates is white, although this does not belong to man insofar as he is man. Now this nature has a twofold existence: one in singulars, the other in the soul; and according to each, accidents follow upon the said nature.

And so in singulars it has a manifold existence according to the diversity of singulars; and yet none of these existences is owed to the nature itself according to its proper, that is, absolute, consideration. For it is false to say that the nature of man, as such, has existence in this singular: if existing in this singular belonged to man insofar as he is man, it would never be outside this singular. Likewise, if not existing in a singular belonged to man insofar as he is man, it would never be in one. But it is true to say that man, insofar as he is man, does not, as such, have it that he is in this singular or in that. It is clear, then, that the nature of man, considered absolutely, abstracts from every existence, yet without exclusion of any of them. And it is this nature, considered in this way, that is predicated of all individuals.

Yet it cannot be said that the notion of the universal belongs to the nature taken in this way, since unity and commonness are part of the notion of the universal. But neither of these belongs to human nature according to its absolute consideration. For if commonness were part of the concept of man, then commonness would be found wherever humanity is found; and this is false, because in Socrates no commonness is found, but whatever is in him is individuated.

And although this nature as understood has the notion of a universal insofar as it is compared to things outside the soul, since it is one likeness of all of them, yet insofar as it has existence in this intellect or that, it is a particular understood species. And so the mistake of the Commentator, in Book 3 of On the Soul, is clear: he wanted to conclude the unity of the intellect from the universality of the understood form. For the universality of that form does not lie in the existence it has in the intellect, but in its being referred to things as a likeness of things. Similarly, if there were one bodily statue representing many men, it is evident that the image or likeness of the statue would have its own singular existence insofar as it was in this matter, but would have the character of commonness insofar as it was a common representation of many.note

It belongs to human nature, according to its absolute consideration, to be predicated of Socrates; and the notion of species does not belong to it according to its absolute consideration (that is, the consideration of human nature), but is among the accidents that follow it according to a certain existence that it has in the intellect. Therefore the name “species” is not predicated of Socrates, so that one would say “Socrates is a species”. This would necessarily happen if the notion of species belonged to man according to the existence he has in Socrates, or according to his absolute consideration, that is, insofar as he is man. For whatever belongs to man insofar as he is man is predicated of Socrates. And yet being predicated belongs to the genus per se, since it is placed in its definition. For predication is something completed by the action of the intellect composing and dividing, yet having a foundation in reality: namely the very unity of those things one of which is said of the other.

Hence the notion of predicability can be included in the notion of this intention that is the genus, which likewise is completed by the action of the intellect. Nevertheless, that to which the intellect attributes the intention of predicability, composing it with something else, is not the intention of genus itself, but rather that to which the intellect attributes the intention of genus, such as what is signified by the name “animal”. Thus, then, it is clear how essence or nature stands to the notion of species. The notion of species is not among the things that belong to it according to its absolute consideration, nor among the accidents that follow it according to the existence it has outside the soul, such as whiteness or blackness; but it is among the accidents that follow it according to the existence it has in the intellect. And in this way the notion of genus or difference belongs to it as well.

Hence in every understanding substance there must be complete freedom from matter, such that it neither has matter as a part of itself nor is like a form impressed on matter, as is the case with material forms.

Hence in the commentary on the ninth proposition of The Book of Causes it is said that an intelligence is something having form and existence; and form is taken there for the simple quiddity or essence itself. How this comes about is plain to see. For whenever things are so related to each other that one is the cause of the other’s existence, the one that has the character of cause can have existence without the other, but not conversely. Now the relation of matter and form is found to be such that form gives existence to matter. And so it is impossible for there to be any matter without form, but it is not impossible for there to be some form without matter.note

For form, as form, has no dependence on matter. But if some forms are found that cannot exist except in matter, this happens to them because they are distant from the first principle, which is first and pure act. Hence those forms that are nearest to the first principle are forms subsisting per se without matter; for it is not form in its whole genus that needs matter, as has been said. And forms of this kind are the intelligences. And so the essences or quiddities of these substances need not be anything other than the form itself.

But the essence of a simple thing, which is its form, can be signified only as a whole, since there is nothing there besides the form that might, as it were, receive the form. And so, in whatever way the essence of a simple substance is taken, it is predicated of that substance. Hence Avicenna says that the quiddity of a simple substance is the simple thing itself, because there is nothing receiving it. The second difference is that, because the essences of composite things are received in designated matter and multiplied according to its division, it happens that some things are the same in species and different in number. But since the essence of simple things is not received in matter, there can be no such multiplication there; and so there need not be many individuals of one species among those substances. Rather, there are as many species as there are individuals, as Avicenna expressly says.

Although substances of this kind are forms without matter, there is not complete simplicity in them, nor are they pure act; they have an admixture of potency. This is clear as follows. Whatever is not part of the concept of an essence or quiddity comes from outside and enters into composition with the essence; for no essence can be understood without the things that are parts of the essence.

But if we posit some thing that is existence alone, such that existence itself is subsistent, this existence will not receive the addition of a difference, because then it would no longer be existence alone, but existence and some form besides. Much less will it receive the addition of matter, because then it would be existence that is not subsistent but material. Hence it remains that there can be only one such thing, one that is its own existence. So in every other thing besides it, its existence must be one thing, and its quiddity, or nature, or form, another. Hence in the intelligences there must be existence besides form; and that is why it was said that an intelligence is form and existence.

And since, as has been said, the quiddity of an intelligence is the intelligence itself, its quiddity or essence is the very thing that it is, and its existence, received from God, is that by which it subsists in reality. For this reason some say that substances of this kind are composed of “that by which it is” and “that which is”, or of “that by which it is” and essence, as Boethius says in his commentary on Book 1 of the Categories. And since potency and act are posited in the intelligences, it will not be difficult to find a multitude of intelligences, which would be impossible if there were no potency in them. Hence the Commentator says in Book 3 of On the Soul that if the nature of the possible intellect were unknown, we could not find multitude in separate substances.

Their distinction from one another, therefore, is according to the degree of potency and act: a higher intelligence, which is nearer to the first, has more of act and less of potency, and so on with the others. And this comes to completion in the human soul, which holds the lowest rank among intellectual substances. Hence its potential intellect stands to intelligible forms as prime matter, which holds the lowest rank in sensible existence, stands to sensible forms, as the Commentator says in Book 3 of On the Soul. And so the Philosopher compares it to a blank tablet on which nothing has been drawn.

And for this reason, because it has more potency than the other intelligible substances, it comes so close to material things that a material thing is drawn to participate in its existence, so that from soul and body there results one existence in one composite—although that existence, as it belongs to the soul, does not depend on the body. And so after this form, which is the soul, there are found other forms that have more potency and are closer to matter, so much so that their existence is not without matter. Among these too there is an order and gradation, down to the first forms of the elements, which are closest to matter. Hence they have no operation except as required by the active and passive qualities and the other things by which matter is disposed to form.

For this existence which God is has this condition: no addition can be made to it. Hence through its very purity it is an existence distinct from every existence. For this reason it is said in the commentary on the ninth proposition of The Book of Causes that the individuation of the first cause, which is existence alone, is through its pure goodness. Common existence, on the other hand, just as it does not include any addition in its concept, so neither does it include in its concept any exclusion of addition; for if it did, nothing could be understood to exist in which something was added over and above existence.

In a second way essence is found in created intellectual substances, in which existence is other than their essence, although the essence is without matter. Hence their existence is not absolute but received, and therefore limited and finite according to the capacity of the receiving nature; but their nature or quiddity is absolute, not received in any matter. And so it is said in The Book of Causes that the intelligences are finite from above and infinite from below. For they are finite as to their existence, which they receive from above; but they are not limited from below, because their forms are not limited to the capacity of some matter receiving them. And in such substances there is no multitude of individuals in one species, as has been said, except in the human soul, because of the body to which it is united.

And although its individuation depends on the body as its occasion, as to its beginning—for it acquires individuated existence only in the body of which it is the act—it does not follow that its individuation perishes when the body is destroyed. For since it has an absolute existence, once individuated existence has been acquired for it by its being made the form of this body, that existence always remains individuated. And so Avicenna says that the individuation and multiplication of souls depends on the body as to its beginning, but not as to its end. And since in those substances quiddity is not the same as existence, they can be ordered in a category; and for this reason genus, species and difference are found in them, although their proper differences are hidden from us.

For in sensible things too the essential differences themselves are unknown to us; hence they are signified through accidental differences that arise from the essential ones, as a cause is signified through its effect—as “two-footed” is given as the difference of man. But the proper accidents of immaterial substances are unknown to us; hence their differences cannot be signified to us either per se or through accidental differences.

But since spiritual substances are simple quiddities, the difference in them cannot be taken from what is a part of the quiddity, but from the whole quiddity. And so Avicenna says in Book 1 of his On the Soul that only species whose essences are composed of matter and form have a simple difference. Likewise in them the genus too is taken from the whole essence, though in a different way. For one separate substance agrees with another in immateriality, but they differ from one another in degree of perfection, according to their distance from potentiality and their nearness to pure act. And so in them the genus is taken from what follows them insofar as they are immaterial, such as intellectuality or something of the kind; and the difference is taken from what follows their degree of perfection—a difference, however, unknown to us.

Nor need these differences be accidental on the ground that they are according to greater and lesser perfection, which does not diversify species. For a degree of perfection in receiving the same form does not diversify the species, as with the whiter and the less white in participating whiteness of the same species. But different degrees of perfection in the forms or natures participated do diversify the species, as nature proceeds by degrees from plants to animals through certain things that are intermediate between animals and plants, according to the Philosopher in the book On Animals. Nor again is it necessary that the division of intellectual substances always be through true differences, because this is impossible to achieve in all things, as the Philosopher says in Book 2 of On Animals.

In a third way essence is found in substances composed of matter and form, in which existence is both received and finite, because they have existence from another; and again their nature or quiddity is received in designated matter. And so they are finite both from above and from below; and in them, because of the division of designated matter, a multiplication of individuals in one species becomes possible. How essence stands to the logical intentions in these substances has been said above.

And so neither the substantial form nor matter has a complete essence either, because in the definition of the substantial form there must be placed that of which it is the form. So its definition is by the addition of something outside its genus, as is the definition of an accidental form too. Thus the natural philosopher places body in the definition of the soul, since he considers the soul only insofar as it is the form of a physical body. But there is this great difference between substantial and accidental forms: just as the substantial form does not have per se absolute existence without that to which it comes, so neither does that to which it comes, namely matter. And so from the conjunction of the two there results that existence in which the thing subsists per se, and from them something one per se comes about; for this reason a certain essence results from their conjunction.

Hence form, although considered in itself it does not have the character of a complete essence, is nevertheless part of a complete essence. But that to which an accident comes is a being complete in itself, standing in its own existence; and this existence naturally precedes the accident that supervenes. And so the supervening accident, by its conjunction with that to which it comes, does not cause that existence in which the thing subsists, through which the thing is a being per se. Instead it causes a certain secondary existence, without which the subsisting thing can be understood to exist, as what is first can be understood without what is second, or the predicate without the subject.

Hence from accident and subject there comes about not something one per se, but something one accidentally; and so from their conjunction no essence results, as it does from the conjunction of form with matter. For this reason an accident neither has the character of a complete essence nor is part of a complete essence; but as it is a being in a qualified sense, so too it has essence in a qualified sense. But what is said most fully and most truly in any genus is the cause of what comes after it in that genus—as fire, which is at the extreme of heat, is the cause of heat in hot things, as is said in Book 2 of the Metaphysics. Therefore substance, which is first in the genus of being and has essence most fully and most truly, must be the cause of accidents, which participate the notion of being secondarily and, as it were, in a qualified sense.

This happens, however, in different ways. For since the parts of substance are matter and form, some accidents follow principally upon form, and some upon matter. Now there is a form whose existence does not depend on matter, such as the intellectual soul; but matter has existence only through form. Hence among the accidents that follow form there is something that has no share in matter, such as understanding, which does not take place through a bodily organ, as the Philosopher proves in Book 3 of On the Soul. But some of those that follow form do share in matter, such as sensing. No accident, however, follows matter without sharing in form.

The accidents that follow form, on the other hand, are proper attributes either of the genus or of the species; hence they are found in all things that participate the nature of the genus or species. Thus the capacity to laugh follows form in man, because laughter comes from some apprehension of the human soul. It should also be known that accidents are sometimes caused by essential principles according to perfect act, as heat in fire, which is always actually hot. But sometimes they are caused only according to an aptitude, and the accidents receive their completion from an external agent, as transparency in the air, which is completed by an external luminous body. In such cases the aptitude is an inseparable accident; but the completion that comes from some principle outside the essence of the thing, or that does not enter into the constitution of the thing, is separable, as being moved and the like.

It should be known, moreover, that genus, difference and species are taken in accidents in a different way than in substances. For in substances something one per se comes about from matter and substantial form, a single nature resulting from their conjunction, which is properly placed in the category of substance. Hence in substances the concrete names that signify the composite are properly said to be in a genus, as species or genera—such as “man” or “animal”. But neither form nor matter is in the category in this way, except by reduction, as principles are said to be in the genus of the things they are principles of. But from accident and subject nothing one per se comes about; hence from their conjunction there does not result any nature to which the intention of genus or species could be attributed.

Hence the names of accidents taken concretely, such as “white” or “musical”, are not placed in the category as species or genera, except by reduction, but only as they are signified in the abstract, as whiteness and music. And since accidents are not composed of matter and form, the genus in them cannot be taken from matter and the difference from form, as in composite substances. Rather, the first genus must be taken from the very mode of existing, according as being is predicated in different ways, by priority and posteriority, of the ten genera. Thus quantity is so called from its being the measure of substance, and quality as it is the disposition of substance, and so on with the others, according to the Philosopher in Book 2 of the Metaphysics. Their differences, however, are taken from the diversity of the principles by which they are caused.

And since proper attributes are caused by the proper principles of the subject, the subject is placed in their definition in place of the difference, if they are defined in the abstract, according as they are properly in the genus: as it is said that snubness is the curvature of the nose. But it would be the other way round if their definition were taken as they are spoken of concretely. For then the subject would be placed in their definition as the genus, since they would then be defined in the manner of composite substances, in which the notion of the genus is taken from matter—as we say that the snub is a curved nose. It is similar if one accident is the principle of another, as action, passion and quantity are the principle of relation; and so the Philosopher divides relation according to these in Book 5 of the Metaphysics.

But since the proper principles of accidents are not always evident, we sometimes take the differences of accidents from their effects. Thus “concentrating” and “dispersing” sight are called the differences of color; these are caused by the abundance and scarcity of light, from which the different species of color are caused. Thus, then, it is clear how essence is in substances and accidents, how it is in composite and simple substances, and how the universal logical intentions are found in them—with the exception of the first principle, which is of infinite simplicity, and to which the notion of genus or species does not belong, nor consequently definition, because of its simplicity. And in this let our discourse find its end and completion.note