Thomas Aquinas: On the Principles of Nature (De Principiis Naturae)

Translation © 1995 by Gerard Campbell

Notice carefully that some thing can be although it is not, whereas another thing truly is. That which can be is called being in potency; that which already is, is called being in act. But there are two kinds of being: namely essential being or the substantial being of the thing (as, for example, to be a man) and this is to be absolutely. The other kind of being is accidental being (as, for example, to be a white man), and this is to be after-a-fashion or in some qualified way.

To each of these kinds of being (i.e. substantial being and accidental being) there is something which is in potency. For there is something which is in potency to be a man, namely, the sperm and the menstrual blood; and there is also something which is in potency to being white, namely, a man. And that which is in potency to substantial being just as much as that which is in potency to accidental being can be called matter; as, for example, the sperm is with respect to the man and the man, in respect to whiteness. But there is a difference. The matter which is in potency to substantial being is called the matter 'out of which' whereas the matter which is in potency to accidental being is called the matter 'in which'. Again, in a strict way of speaking, that which is in potency to substantial being is called prime matter, however, that which is in potency to accidental being is called the subject: This is why it is said that accidents are in a subject--and also why it is not said that substantial form is in a subject. And it is according to this notion that 'matter' differs from 'subject' because the subject does not have its being from what comes to it but has in itself complete being (as, for example, a man does not derive his being from his whiteness). But matter, on the other hand, does derive its being from what comes to it, since, of itself, it has incomplete being. Therefore, strictly speaking, (substantial) form gives being to matter, whereas accidents do not give being to a subject, but rather, the subject to the accidents. Sometimes, however (in popular terminology), one term is used for the other, that is, 'matter' for 'subject' and vice-versa.

Again, just as everything which is in potency can be called 'matter' in the same way everything by which something has being (either substantially or accidentally), can be called 'form'; for example, in man since he is potentially white, he becomes actually white through whiteness, and the sperm, since it is potentially man becomes actually man through the soul. And because it is the form which makes something to be in act, accordingly the form is called 'act'. Moreover, that which brings about substantial being in act is called 'substantial form', and that which brings about accidental being in act is called 'accidental form'.

Now since generation (coming-to-be) is a movement towards form, there are two kinds of generation which correspond to these two kinds of forms: absolute coming-to-be corresponds to substantial form-relative coming-to-be (coming-to-be-in-a-certain-way), corresponds to accidental form. For whenever substantial form is introduced, we say that something absolutely becomes (i.e. without qualification)--for example, a man becomes a man or a man comes-to-be. However, when accidental form is introduced, we do not say that something becomes absolutely--but rather that it becomes this--for example, when a man becomes white we do not say that he becomes a man or that a man has come-to-be but rather that the man becomes white or comes-to-be white. And there is a two-fold notion of corruption (passing away) which is opposed to this two-fold notion of generation (coming-to-be)--namely, absolute passing-away and relative passing away. Absolute generation (coming-to-be) and absolute corruption (passing away) exist only within the genus of substance. On the other hand relative generation and relative corruption (coming-to-be-and passing-away-in some way or other) are found in all of the other genera. And because generation is a kind of movement from non-being to being and, conversely, corruption from being to non-being, it is not from just any kind of non-being that generation comes but from non-being which is being in potency--just as, for example, the statue comes from bronze which is statue in potency, not in act.

And, therefore, in order that generation (coming-to-be) come about, three things are necessary: namely, a potential something which is the matter, and not-being in act which is the privation, and that through which it becomes in act, namely, the form. Let us take an example: when a statue-is made from bronze, the bronze which is in potency for the form of the statue is 'the matter'; the privation is the shapelessness or the lack of the form (of statue in the molten bronze); the shape by which we call it a statue is the form. The form of statue, however, is not the substantial form, because the bronze before the coming-to-be of this form (of statue) has being in act, and its being does not depend upon this shape (of statue) which is an accidental form. All artificial forms are accidental forms. For art only operates upon those things already constituted in being by nature.

There are, therefore, three principles of nature, and these are matter, form, and privation. One of these principles, namely, the form, is that towards which coming-to-be moves: the other two are on the side of that from which coming-to-be arises. Hence matter and privation are in the same subject but seen from different viewpoints. For it is the same subject which is both bronze and unshaped (with respect to the statue) before the advent of the form--but it is for one reason that we call it bronze and for another reason that we call it unshaped. Hence privation is called a principle, not essentially but accidentally, because it coincides with the matter; for example, we say that a doctor builds something accidentally, for the doctor builds something not in so far as he is a doctor, but in so far as he is a builder and builder coincides with doctor in one subject. Now accidents are of two kinds. There are necessary accidents which are not separated from the thing, as, for example, risibility from man; and there are non-necessary accidents which are separated, such as whiteness from man. So although privation is an accidental principle, it does not follow that it is not necessary for coming-to-be. For matter is never without privation: for in so far as the matter is under one form it has the privation of another form and viceversa. For example, in fire there is the privation of air and in air the privation of fire.

It should be noticed that although coming-to-be arises from not-being, we do not say that negation is its principle, but rather, privation; and this is because negation does not determine the subject for itself. Not-seeing can be said even of non-beings as, for example, "Chimeras do not see." Likewise it can be said of beings which are not intended by nature to have sight, such as a stone. But privation is said only of a determinate subject which is intended by nature to have a given state: thus blindness, for example, is only said of those things which are intended by nature to see. And because coming-to-be does not arise from absolute non-being but from non-being which is in a certain subject, and not in just any subject but in a determined one, (for it is not from just any non-burning thing that fire comes about, but from a non-burning thing which by nature can become the form of fire)--it is because of this that privation is a principle. But in this, privation as a principle differs from the other principles because the others are principles both in being and becoming. For that this should become a statue it is necessary that there be bronze and, ultimately, the shape of the statue: and, moreover, once it is a statue, it is again necessary that both of these exist. But privation is a principle in becoming and not in being, for while the statue is becoming it cannot be a statue. If it were a statue, it could not be becoming one, because what is becoming is only in successive stages, as are time and motion. But from that which already is a statue, there is not the privation of statue in it. Because affirmation and negation cannot exist simultaneously, similarly neither can the state and the privation of that state. And so privation is an accidental principle in the sense explained above; the other two are essential principles.

From what has already been said, it is evident that matter differs from form and from privation according to definition. For matter is that in which both form and privation are understood, just as both a shape and a lack of a shape are understood in the bronze. And sometimes the way in which we name the matter involves the notion of privation and sometimes it does not involve the notion of privation; for example, bronze, although it is the matter of the statue does not involve the notion of privation because when I say "bronze", the lack of a form or the lack of a shape is not included in my concept. On the other hand, flour, since it is the matter with respect to bread does involve in itself the privation of the form of bread, because when I say "flour" the lack of form or the disorganization opposed to the form of bread is signified. And because in coming-to-be the matter or the subject remains, but the privation does not, and neither does the composite of matter and privation, accordingly, matter which does not involve the notion of privation remains; however, that matter which does involve the notion of privation, is transitory.

It should also be noted that some matter has a composition of form; for example, although the bronze is the matter with respect to the statue, nevertheless, the bronze itself is a composite of matter and form. Therefore, bronze cannot be called prime matter because it has a matter. Only that matter which can be understood without any form and privation, but which is the subject of both form and privation, is called prime matter because there is not any other matter prior to it. And this prime matter is also called "hyle".

Because all knowledge and every definition is through the form, therefore, prime matter cannot be known or defined by itself, but by the composite: and so we say that prime matter is that which is related to all forms and all privations like the bronze is related to the statue and to the privation of some figure. And prime matter here means prime in an absolute way. For something can also be called primary with respect to a certain genus, such as water being the prime matter in the genus of liquids. Nevertheless, water is not prime in an absolute way, because it is a composite of matter and form; hence it has a matter prior to it.

Again it should be noted that both prime matter and form neither come-to-be nor pass-away because all coming-to-be is from something to something. Now that from which the coming-to-be is from is the matter; and that to which the coming-to-be is to is the form. Therefore, if the matter or the form also come-to-be, there would be a matter of the matter and a form of the form ad infinitum. Hence, strictly speaking, only the composite comes-to-be.

Again it should be noted that prime matter is said to be numerically one in all things. But "numerically one" can be said in two ways: first of all, of that which has a one determined form in number as, for example, Socrates. Now prime matter is not said to be numerically one in this way since, in itself, it does not have any one form. Secondly, something can also be said to be numerically one which is without the dispositions which would make it differ according to number. And it is in this way that prime matter is said to be numerically one, because its concept involves the lack of all of the dispositions which account for differentiation in number.

And finally it should be noted that although the concept of prime matter does not include any form or privation, just as in the concept of bronze there is included neither a shape nor the lack of a shape, nevertheless prime matter is never without some form and privation. For sometimes it is under one form and sometimes under another form. But prime matter cannot exist by itself alone, because by its very definition it does not have any form, and so does not have any actual existence, sincebeing in act is only through the form. But prime matter is only in potency. And, therefore, anything whatsoever that exists in act cannot be called prime matter.

From what has been said it is evident that there are three principles of nature, namely, matter, form, and privation. But these three are not sufficient to explain coming-to-be. For whatever is in potency cannot reduce itself to act: for example, the bronze which is a statue in potency does not make itself a statue, but requires an agent which draws out from potency to act the form of the statue. Neither can the form draw itself into act from potency. And here I am speaking of the form generated which we call the term of generation. For the form does not exist unless it exists in fact: the agent, however, exists in becoming, that is, while the thing is coming-to-be. Besides the matter and the form, therefore, there must exist some other principle which acts, and this is called the efficient cause or, the moving cause, or the agent, or the principle from whence the movement exists.

And because, as Aristotle points out in Bk. II of the Metaphysics, everything which acts, acts only when intending something, a fourth principle must also be posited--namely, that which is intended by the agent, and this is called the end. And it should be noted that although every agent whether natural or voluntary intends an end, nevertheless it does not follow that every agent knows the end or deliberates about the end. For to know the end is necessary only in those things whose actions are not determined but which can move towards opposites, as is the case with voluntary agents; for these (voluntary agents) it is necessary that they know the end through which they determine their actions. In natural agents, however, their actions are determined, hence it is not necessary to choose those things (means) which are for the end. We may use the example given by Avicenna of the cithara (a musical instrument resembling a lyre) player who does not have to deliberate about each note of a chord since the notes are determined within it, for should someone deliberate, there would be a delay between the notes which would be discordant. All of this can be seen more clearly with respect to voluntary agents who deliberate than with natural agents--and, accordingly, it is evident a fortiori that if the voluntary agent whose deliberation is more evident, does not always deliberate, then certainly neither does the natural agent. It is therefore possible for a natural agent to intend an end without deliberation and this intending is nothing other than having a natural inclination towards something.

From what has been said it is evident that there are four causes, namely, the material, the efficient, the formal, and the final. Now although principle and cause are spoken of in a quasi-convertible way, as is said in Bk. V of the Metaphysics, nevertheless, in the Physics, Aristotle sets down four causes and three principles. Furthermore, he accepts as causes just as much extrinsic as intrinsic ones. Matter and form are called intrinsic causes of the thing due to the fact that they are the constitutive parts of the thing. The efficient and the final causes are called extrinsic causes because they are outside of the thing. But he accepts only the intrinsic causes as principles. Moreover, privation is not named among the causes because it is an accidental principle, as was said earlier. And when we speak of the four causes, we mean the essential causes to which, however, accidental causes are reduced because everything which is accidental is reduced to that which is essential.

But although Aristotle considers principles as intrinsic causes in Bk. I of the Physics, nevertheless, as he says in Bk. XI of the Metaphysics (this is today Bk. XII, 11 70b22-30), "principle" is properly said of extrinsic causes and "elements" of those causes which are the parts of the thing, that is, of the instrinsic causes. However "cause" is used with respect to both, although sometimes one term is used for the other; for every cause can be called a principle and every principle, a cause. But, the notion of cause seems to add something over and above what is commonly called principle, because that which is first, whether or not something posterior follows from it, can be called a principle, just as the artisan is called the principle of the knife because a knife comes into being as a result of his activity: but when something is moved from blackness to whiteness, blackness is called the principle of that movement (and universally, everything from which a movement begins is called a principle), nevertheless, blackness is not that from which the being of whiteness follows. But "cause" is said only of that thing which is first from which a posterior thing follows in being (that is, with real dependence). Hence, a cause is that from the being of which another being follows. Accordingly, that which is first from which the movement begins cannot be called a cause essentially (per se) even if it is called a principle. This is the reason privation is placed among the principles but not among the causes, because privation is that from which the coming-to-be begins. But privation can also be called a cause per accidens, in so far as it coincides with the matter, as was explained earlier.

Moreover, the term "element" is used properly only of the causes from which the composition of the thing results--which, properly speaking, are material causes. And again, not from just any material cause whatsoever but from that from which results the primary composition of the thing; for example, we do not say that the limbs are the elements of the man because the limbs are themselves composed of other things. But we do call earth and water elements because these are not composed of other bodies--but from these elements results the primary composition of natural bodies. Thus Aristotle in Bk. V of the Metaphysics says that "an element is that from which a thing is primarily composed, and is in the thing, and is not divided according to the form". The explanation of the first part of this definition namely, "that from which a thing is primarily composed", is already clear from what has been said above. The second part, namely, "and is in the thing", is placed in the definition in order to differentiate (the element from) that other matter (prime matter) which totally passes-away through coming-to-be: for example, the bread is the matter of blood, but blood only comes-to-be if the bread passes-away; hence the bread does not remain in the blood and so the bread cannot be called an element of blood. But for it to be an element it must remain in some way, since it does not entirely pass-away, as is said in the treatise On Generation. The third part of the definition, namely, "and is not divided according to the form" is placed in the definition to differentiate the element from those things which have diverse parts in form, that is, in species; for example, the hand, whose parts are flesh and bone, but which differ according to species; for example, any part whatsoever of water is water. It is not necessary to the being of the element to be not divided according to quantity, it is sufficient if it be not divided according to species. If something is also not divided in any way, it is called an element, as letters are said to be the elements of words. Accordingly, it is evident from what has been said that "principle" in some way in which it is used implies more than "cause", and "cause", more than "element"--and this is what the Commentator (Averroes) says in his commentary on Bk. V of the Metaphysics.

Having seen that there are four genera of causes, it should be noted that it is not impossible for a same thing to have several causes, such as the statue whose cause is bronze and the artisan, but the artisan as the efficient cause, bronze as the material cause. Nor is it impossible that a same thing be the cause of contraries, just as the helmsman is the cause of the safety of the ship and of its sinking; but of the latter by his absence, and of the former by his presence.

It should also be noted that it is possible that a same thing be cause and caused with respect to the same thing but in different ways: for example, walking is the cause of health in the way of efficient causality, but health is the cause of walking in the way of final causality, for sometimes walking is done for the sake of health. Another example is that the body is the matter of the soul, while the soul is the form of the body. The efficient cause is called a cause with respect to the end, for the end can only exist in act through the operation of the agent: but the end is called the cause of the efficient cause, for the efficient cause operates only through the intention of the end. Hence the efficient cause is cause of that which is the end, as, for example, walking for the sake of health; nevertheless the efficient cause does not make the end, that is, it does not make the end to be a final cause. A doctor, for example, makes health to be in act, nevertheless, he does not make health be an end. The end, however, is not the cause of that which is the efficient cause, but it is the cause that an efficient cause be an efficient cause. For health does not make the doctor to be a doctor (and I am speaking of the health resulting from the operation of the doctor) but it does make the doctor be an efficient cause. Similarly the end makes the matter be the material cause and the form be the formal cause, since the matter receives the form only for an end, and the form perfects the matter only for and end. Hence the end is called the cause of causes because it is the cause of the causality in all causes. Also matter is called the cause of the form in so far as the form exists only in matter; and similarly the form is the cause of the matter, in so far as matter has being in act only through the form. For matter and form are said to be mutually related as is said in Bk. II of the Physics. For they are said in relation to the composite, as are parts in relation to the whole, and the simple in relation to the composite.

But because every cause, in so far as it is a cause, is naturally prior to what is caused, it should be noted that "prior" is said in two ways, as Aristotle says in Bk. XVI of On Animals (c.f. On the Generation of Animals II, 6, 742a21). Through this diversity, something can be called both prior and posterior and cause and caused with respect to the same thing. For something is called prior to another in generation and time or prior in substance and completeness. Therefore, since the operation of nature is from the imperfect to the perfect and from the incomplete to the complete, the imperfect is prior to the perfect according to generation and time, but the perfect is prior to the imperfect according to substance: for example, it can be said that the man is prior to the boy in substance and completeness, but the boy is prior to the man in generation and time. But, although in things capable of coming-to-be, the imperfect is prior to the perfect and potency is prior to act--considering that in any subject that what is prior is imperfect rather than perfect and in potency rather than in act--nevertheless, absolutely speaking, it is necessary that what is in act and perfect be prior; because what reduces potency to act is in act and what perfects the imperfect is perfect. The matter is prior to the form in generation and time, for that to which something comes is prior to that which comes to it. But the form is prior to the matter in substance and complete being because the matter has complete being only through the form. Similarly the efficient cause is prior to the end in generation and time, since the movement towards the end is brought about by the efficient cause; but the end is prior to the efficient cause, in so far as it is efficient cause, in substance and completeness, since the action of the efficient cause brings about completeness only through the end. Accordingly, these two causes, the material and the efficient are prior by way of generation; but the form and the end are prior by way of perfection.

And it should be noted that there are two kinds of necessity, namely, absolute necessity and conditional necessity. That necessity is absolute which proceeds from prior causes in the order of generation, and these are the material and the efficient causes; for example the necessity of death which comes about from matter, namely from the disposition of contrary components--and it is called absolute because there is no impediment to it. This necessity is also called the necessity of matter. On the other hand, conditional necessity proceeds from causes which are posterior in generation, namely, from the form and the end; for example, we say that it is necessary that there be conception if a man is to be generated. And this necessity is also called conditional, because it is not absolutely necessary that this woman conceive but only under this condition, namely, if a man is to be generated. And this necessity is called the necessity of the end.

And it should be noted that three of the causes, namely, the form, the end and the efficient cause, can coincide in one thing as is evident in the coming-to-be of fire. For fire brings fire to be, therefore fire is the efficient cause in so far as it brings to be; and again, fire is the form in so far as it makes to be in act what before was in potency; and again it is the end in so far as it is the intention of the agent and in so far as the operation of the agent is terminated in it. But there are two kinds of ends, namely, the end of the generation and the end of the thing generated, such as is evident in the generation of a knife: for the form of a knife is the end of the generation, but cutting, which is the operation of the knife, is the end of the thing generated, that is, the knife. However, sometimes the end of the generation coincides with the other two above-mentioned causes (the form and the efficient cause), namely, when the generation is of what is similar in species; for example, when a man generates a man or an olive tree generates an olive tree which (coincidence of form, efficient cause, and end) cannot be understood of the end of the thing generated. Nevertheless it should be noted that the end is identical with the form numerically, because it is the same something numerically which is the form generated and is the end of generation. But the end of the generation is not identical with the efficient cause in the same number but in the same species. For it is impossible that the maker and the thing made be numerically the same, but they can be specifically the same; for example, when a man generates a man, the man generating and the man generated are different by number but of the same species. However, the matter cannot coincide with the other causes because the matter by the fact that it is a being in potency, has the notion of imperfection; but the other causes, since they are in act, involve the notion of perfection; moreover, the perfect and the imperfect do not coincide in the same thing.

Accordingly, having seen that there are four causes, namely, the efficient, the material, the formal, and the final it should be known that these same causes are divided in many ways. There can be prior cause and posterior cause, as when we say that both the art and the doctor are the cause of health, but the art is the prior cause and the doctor the posterior cause. And the same division holds for the formal cause and the other causes. And notice carefully that we should always lead back a question to the first cause. For example, if we ask "why is this man healthy?" the answer is "Because the doctor healed him" And so we should ask again, "By what means did the doctor heal him?"--"Through the act of healing which he possesses."

It should also be noted that posterior cause is also called proximate cause and prior cause, remote cause. Hence these two divisions of causes--prior and posterior, remote and proximate--signify the same thing. However, it should be observed that always what is more universal is called remote cause, what is more particular is called proximate cause. For example, we say that the proximate form of man is his definition, namely, rational mortal animal; but animal is more remote and substance again more remote. And similarly the proximate matter of the statue is bronze, but the remote matter is metal and the again more remote is body.

Again, (according to another division) of causes, some are essential and some are accidental. A cause is called essential which is the cause of something in so far as it is this kind of thing--for example, the builder is the cause of the house and the wood is the matter of the bench. A cause is called accidental which happens (to coincide with) an essential cause-for example, when we say that the grammarian; builds (a house). For the grammarian is called a cause of the building accidentally, for he does so not in so far as he is grammarian, but in so far as he is builder and it happens that he be a grammarian. And the case is similar in the other causes.

Again, (according to another division) of causes, certain ones are simple, others are composite. A cause is called simple when what is the essential cause is alone called cause or also when what is the accidental cause is alone called cause--for example if we say that the builder is the cause of the house and, similarly, if we say the doctor is the cause of the house. However a cause is called composite when both are said to be causes--for example, if we say the doctor-builder is the cause of the house. A cause can also be called simple in the way in which Avicenna explains it: that which causes without being united to another, as the bronze of statue--for the statue is of bronze without the addition of any other matter--and just as it is said that the doctor produces health or that fire produces heat. However he calls a cause composite when many things must come together in that which is a cause, just as one man is not the cause of the movement of the ship but many and as one stone is not the matter of the house, but many.

Again, (according to another division) of causes, some are actual and others are potential. An actual cause is one which actually causes a thing, as the builder when he is building, or the bronze when a statue is made of it. A potential cause is one which, although it is not causing the thing in act, nevertheless can cause it, as the builder while he is not building. And it should be noted that in speaking of actual causes it is necessary that the cause and the caused exist simultaneously, such that if one is, the other must also be. For if a builder be in act, it is necessary that he build. And if there is building in act, there must be a builder in act. But this is not necessary in causes which are only potential. And it should be noted moreover that a universal cause is related to a universal effect, while a singular cause is related to a singular effect, just as we say that builder is the cause of house and this builder is the cause of this house.

It should also be noted that in speaking of the intrinsic principle, namely, matter and form, there is an agreement and difference of principles according to the agreement and difference of what results from the principles. For certain things are numerically the same, such as Socrates and this man (in pointing at Socrates). Other things are numerically diverse but the same in species, such as Socrates and Plato, who although they agree as men (human species) nevertheless, differ by number. Also certain things differ according to species but we are the same according to genus; for example, man and ass are both in the genus animal. Again certain things are diverse in genus but are the same only according to analogy; such as substance and quantity, which do not agree in any genus but which are brought together only according to analogy. For they are found together only in that which is being; being, however is not a genus, since it is not predicated univocally, but analogously.

In order to understand this, however, it should be known that something is predicated of many in three ways: univocally, equivocally and analogously. A univocal predication occurs when one thing is predicated according to the same name and according to the same nature, that is, the same definition, as "animal" is predicated of "man" and "ass". For both of these are called "animal" and each of them is a living substance capable of sensation, which is the definition of animal. Equivocal predication occurs when something is predicated of others according to the same name and according to different natures, as "dog" is said of a "barking animal" and a "steller constellation", which things agree only in name and not in definition or signification; for that which is signified by the name is the definition, as is said in Bk. IV of the Metaphysics. An analogical predication occurs when one thing is predicated of many, which are of different natures, but of which one some thing is attributed to them, as "healthy" is said of the "animal body", and of "urine", and of the "medicine" but "healthy" does not signify exactly the same thing in all of them. For "healthy" is said of "urine" as of a sign of good health, of "the body", as of its subject, of "the medicine" as of a cause. Nevertheless, all of these natures are attributed to a one end, namely, health. For sometimes those things which are brought together according to analogy, that is in proportion, or comparison, or agreement, are attributed to one end, as is evident in the above example; sometimes in one agent, as "doctor" is said both of one who operates through art and of one who operates without art, such as the midwife--and the same also holds for instruments, but through attribution to a one agent which is the art of medicine. Also sometimes the analogy is based through attribution to a one subject as when "being" is said of substance and of quantity and of quality, and of the other predicaments. For it is not for exactly the same reason that substance is said of being, and quantity, and the others--but all of them are called being by the fact that they are attributed to substance, which is the subject of the others. And, therefore, being is said primarily of substance and secondarily of the others. Accordingly, being is not the genus of substance and quantity because no genus is predicated primarily and secondarily of its species. Being is predicated analogously. And it is in this light that we say that substance and quantity differ in genus, but are the same according to analogy.

Wherefore, of those things which are numerically one, both the form and the matter are numerically one, as in the case of Tullius and of Cicero. Moreover, of those things which are the same specifically, but differ by number, both the matter and the form are not the same numerically but specifically as in the case of Socrates and of Plato. And similarly, of those things which are the same generically, their principles are generically the same, as in the case of the soul and the body of an ass and of a horse which differ specifically but are the same generically. And it is also similar for those things which agree only according to analogy, for their principles are similar only according to analogy or proportion. For matter, form, and privation, or potency and act are principles of substance and of the other genera. Nevertheless, the matter of substance and of quantity (and similarity with respect to form and privation) differ generically, but agree only according to a proportion which consists in this--just as the matter of substance is related to substance in the nature of matter, so is the matter of quantity related to quantity. However, just as substance is the cause of all the other genera, so the principles of substance are the principles of all the other genera.

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