Note that something can be, even if it is not, while something [simply] is. That which only can be [but is not] is said to be in potentiality; that which already is is said to be in actuality. But there are two kinds of being, namely the essential, or substantial being of the thing, as for a man to be, and this is just to be, without any qualification. The other kind of being is accidental being, such as for a man to be white, and this is [not just to be, but] to be somehow.
And it is with respect to both kinds of being that something is in potentiality. For something is in potentiality toward being a man, as the sperm and the menstrual blood; and something is in potentiality toward being white, as a man. Both that which is in potentiality in respect of substantial being and that which is in potentiality in respect of accidental being can be said to be matter, as the sperm can be said to be the matter of man and the man the matter of whiteness. But they differ in that the matter that is in potentiality in respect of substantial being is called matter from which [something is made - materia ex qua], while that which is in potentiality in respect of accidental being is called matter of which [something is made - materia in qua].
Again, properly speaking, what is in potentiality toward accidental being is called a subject, while that which is in potentiality toward substantial being is properly called matter. And it is significant that what is in potentiality toward accidental being is called a subject, for we say that an accident is in a subject, while of substantial form we do not say that it is in a subject.
So matter differs from subject in that a subject does not have being from what comes to it, as it has complete being in itself. For example, a man does not have [his] being from [his] whiteness; but matter has being from what comes to it, for matter in itself does not have complete being, indeed, it does not have any being, as the Commentator says in the second book of On the Soul. And so, absolutely speaking, form gives being to matter, but the subject gives being to the accident, even if sometimes the one term is taken for the other, i.e. “matter” for “subject”, and vice versa.
Just as everything that is in potentiality can be called matter, so everything from which something has being, whether accidental or substantial being, can be called a form; just as a man, who is white in potentiality, will be actually white by whiteness, and the sperm, which is a man in potentiality, will be actually a man by the soul. And since form makes something actual, form is also called actuality. That which makes something actual in accidental being is accidental form, and that which makes something actual in substantial being is substantial form.
Since generation is motion towards form, to these two kinds of form there correspond two kinds of generation: to substantial form there corresponds generation absolutely speaking, while to accidental form there corresponds generation with qualification. For when the substantial form is introduced, something is said to come to be, without further qualification. But when an accidental form is introduced, we do not say that something comes to be, without qualification, but that something comes to be this; just as when a man becomes white, we do not say that he comes to be, absolutely speaking, but that he comes to be white. And to these two kinds of generation there correspond two kinds of corruption, namely corruption in an absolute sense, and corruption with qualification. Generation and corruption absolutely speaking are only in the category of substance, while those with qualification are in the other categories.
And since generation is a kind of mutation from non-being into being, and corruption, conversely, should be from being to non-being, generation starts not from just any kind of non-being, but from a non-being that is a being in potentiality: for example, a statue is generated from bronze, which can be a statue, but is not actually a statue.
So for generation three things are required: a being in potentiality, which is matter, non-being in actuality, which is privation, and that by which the thing will be actual, namely form. Just as when from bronze a statue is formed, the bronze, which is in potentiality toward the form of the statue, is matter; its being amorphous is called privation; and its shape, on account of which it is called a statue is its form, though not its substantial form, for the bronze was already actual even before the introduction of this form or shape, and its existence does not depend on this shape, but is an accidental form. For all artificial forms are accidental, because art works only on what is supplied by nature already in complete existence.
So there are three principles of nature, namely matter, form and privation; of which one is that to which generation proceeds, namely form, and the other two are those from which generation proceeds. Therefore, matter and privation are the same in their subject, but differ in their concepts. For the very same thing that is bronze is amorphous before the advent of the form; but it is for different reasons that it is called bronze and amorphous.
Therefore privation is called a principle not per se[in its own right] but per accidens[by coincidence], namely because it coincides with matter, just as we say that this is per accidens: the doctor builds a house, for he builds not insofar as a doctor, but insofar as a builder, who happens to be a doctor.
But there are two kinds of accidents: namely necessary [accident], which is not separated from its subject, as risibility from man, and not necessary [accident], for example, whiteness, which can be separated from man. Therefore, though privation is a per accidens principle, it does not follow that it is not required for generation, because matter is never stripped of privation; for insofar as it is under one form, it has the privation of another, and conversely, as in fire there is the privation of the form of air.
We should know that even if generation proceeds from non-being, we do not say that its principle is negation, but that it is privation, for a negation does not determine its subject. For that it does not see can be said also of non-beings, as [when we say that] a chimera does not see, and also of beings which are naturally inept to have sight, as of stones. But a privation can be said only of a determinate subject, in which the opposite habit is naturally apt to occur, for example, only those can be said to be blind that are naturally apt to see.
And since generation does not proceed from non-being absolutely speaking, but from a non-being in some subject, and not in just any kind of subject, but in a determinate subject (for it is not from just any kind of non-being that fire is generated, but from that kind of non-fire in which the form of fire is apt to come to be), we say that privation is a principle.
But it differs from the others in that the other two are principles both of being and of coming to be. For in order that a statue is generated there has to be bronze, and in the end there has to be the form of the statue, and further when the statue already exists, these two also have to exist. However, privation is only the principle of coming to be, but not of being, for while the statue is still coming to be, it is necessary that the statue does not yet exist. For if it already existed, it would not be coming to be, for what is still coming to be does not yet exist, apart from processes. But when the statue already exists, there is no privation of the shape of the statue, for affirmation and negation cannot stand together, and similarly neither can privation and habit.
Again, privation is a principle per accidens, as was explained above, and the other two are principles per se. From what has been said it is clear, then, that matter differs from form and privation in its concept. For matter is that in which form and privation are thought to be, as it is in the bronze that form and formlessness are thought to be.
Sometimes matter is named with privation, and sometimes without it. For example, the concept of bronze, when it is the matter of the statue, does not imply privation: for when I call something bronze, this does not imply that it is amorphous or formless. On the other hand, the concept of flour does imply the privation of the form of bread, for when I call something flour, this does signify an amorphousness or formlessness opposite to the form of bread.
And since in the process of generation matter or the subject remains, but privation or what is composed of matter and privation does not, that matter which does not imply privation in its concept is permanent, while that matter which does is transient.
We should know that some matter has some form, for example, the bronze, which is matter in respect of the statue, but bronze itself is composed of matter and form; wherefore bronze is not called prime matter, for it has matter. But that matter which is thought of without any kind of form or privation as subject to all forms and privations is called prime matter, because there is no other matter before it. And this is also called hyle.
Now, since [any] definition and cognition is [obtained] by form, prime matter cannot be cognized or defined in itself, only by comparison, as when we say that prime matter is that which is to all forms and privations as bronze is to the form of the statue and to the lack of this form. And this matter is called prime matter without qualification.
For something can be called prime matter in respect of a genus, as water is the prime matter of all liquids. But it is not prime matter without qualification, for it is composed of matter and form, so it has matter prior to it.
We have to know that prime matter, and even form, is not generated, nor corrupted, for every generation proceeds to something from something. That from which generation proceeds is matter, and that to which generation proceeds is form. Therefore, if either matter or form were generated, then matter would have matter and form would have form, and so on, in infinitum. So properly speaking only the composite substance is generated.
We also have to know that matter is said to be numerically one in all things. But something is said to be numerically one in two ways. First, that is said to be numerically one which has one determinate form, e.g., Socrates; but prime matter is not said to be numerically one in this way, for in itself it does not have any form. Second, a thing can also be said to be numerically one because it lacks those dispositions which make things numerically different, and it is in this way that matter is said to be numerically one.
We should know that although matter does not have in its nature some form or privation, as in the concept of bronze neither shape nor the lack of some shape is included; nevertheless, matter is never stripped of form or privation, for sometimes it is under one form, while sometimes it is under another. But in itself it can never exist, since, as in its concept it does not have any form, it does not have actual existence - for something can have actual existence only by its form -, but it is only potentially. So nothing in actual existence can be called prime matter.
From what has been said it is clear, then, that there are three principles of nature, namely matter, form and privation. But these three are insufficient for generation, for nothing drives itself into actuality, e.g., a chunk of bronze, which is in potentiality to become a statue, does not make itself into an actual statue, but it needs a sculptor, who brings out the form of the statue from potentiality to actuality. Neither would the form bring out itself from potentiality to actuality (and I am speaking here about the form of the thing being generated, which we call the end of the generation), for the form does not exist until it has come to be, but what is acting is already existing during the process of generation. So it is necessary to have another principle beside matter and form that acts, and this is called the efficient or moving cause, or the agent or the principle of motion. And since, as Aristotle says in the second book of his Metaphysics, whatever acts does so only with intending something, there has to be also a fourth [principle], namely that which is intended by the agent, and this is called the end.
We have to know, however, that every agent, natural as well as voluntary, intends some end. But from this it does not follow that all agents recognize this end, or deliberate about the end. For to recognize the end is necessary only for those agents whose acts are not determined, but which can have alternatives for [their] action, namely, voluntary agents; and so they have to recognize their ends by which they determine their actions. However, the actions of natural agents are determined, so it is not necessary that they elect the means to an end. And this is what Avicenna illustrates with his example of the guitar player, who need not deliberate any plucking of the strings, because these are determined for him, for otherwise there would be delays between the single sounds, which would result in dissonance.
Now a voluntary agent rather appears to deliberate than a natural agent. Whence by locus a maiori it follows that [if even voluntary agents, who rather appear to deliberate, sometimes do not deliberate their actions, then neither do natural agents. Therefore] it is possible for a natural agent to intend some end without deliberation. And this kind of intending an end is nothing, but to have a natural inclination towards it.
From what has been said, then, it is clear that there are four kinds of causes, namely material, efficient, formal and final. And although the terms “principle” and “cause” can be used interchangeably, as is said in the fifth book of the Metaphysics, in the Physics Aristotle distinguished four causes and three principles. For [there] he took causes to comprise both extrinsic and intrinsic ones. Now matter and form are said to be intrinsic to the thing, for they are constituent parts of the thing; but the efficient and the final cause are said to be extrinsic, for they are outside of the thing. But [in this passage of the Physics] he took only the intrinsic causes to be principles. On the other hand, privation is not counted among the causes, for privation is a per accidens principle, as we said. So when we speak about the four causes, we mean the per se causes, but also the per accidens causes are reduced to the per se ones, for whatever is per accidens is reduced to what is per se.
But even if in the first book of his Physics Aristotle calls the intrinsic causes principles, as he says in the eleventh book of his Metaphysics, properly speaking the extrinsic causes are principles and the intrinsic causes that are parts of the thing are elements, and both can be called causes. But sometimes these terms are used interchangeably. For every cause can be called a principle and every principle can be called a cause, though the concept of a cause seems to add something to that of principle in its ordinary sense, for whatever is first can be called a principle, whether there results some existence from it or not. For example, a craftsman can be called the principle of a knife, as from his work there results the being of the knife. But when something turns from black to white, we can say that blackness is the principle [beginning] of this change - and generally speaking everything from which some change begins can be called a principle - still, from this blackness there did not result the being of whiteness. But only that is called a cause from which there follows the being of a posterior thing; so we say that a cause is from the being of which there results the being of something else.
And so that first from which the motion starts cannot be called a cause per se, even if it is a principle, wherefore privation is posited among principles, though not among causes, for privation is that from which generation starts. But [privation] can also be called a cause per accidens, insofar as it coincides with matter, as was said above. However, only those things are called properly elements that are causes of which the thing is composed, which are properly material, and not just any material causes, but only those of which the thing is primarily composed. We do not say, for example, that his members are the elements of a man, for the members themselves are also composed of others; but we do say that earth and water are elements, for these are not composed of other bodies, but it is from them that all natural bodies are primarily composed. Therefore Aristotle in the fifth book of the Metaphysics says that an element is something from which a thing is primarily composed, is in the thing, and is not divided according to form.
The first particle of this definition, namely, “something from which a thing is primarily composed”, is evident from what has been just said. The second particle, namely, “is in the thing”, is put here to distinguish elements from that kind of matter which is totally corrupted in generation. For example, bread is the matter of blood, but blood is not generated, unless the bread from which it is generated passes away; so the bread does not remain in the blood, whence bread cannot be said to be an element of blood. But elements somehow have to remain, since they do not pass away, as it is said in the book On Coming to Be and Passing Away. The third particle, namely, that an element is not divided according to form, is meant to distinguish an element from those things that have parts different in form, i.e., in species, as, for example, a hand, the parts of which are flesh and bones, which are different in species. But an element is not divided into parts that differ in species, as water, of which every part is water. For it is not required for something to be an element that it should be indivisible in quantity, but it is sufficient, if it is not divisible according to species; but if something is indivisible also in this way, then it is also called an element, as letters are called the elements of expressions. So it is clear that “principle” covers more than “cause”, and “cause” more than “element”. And this is what the Commentator says in the fifth book of the Metaphysics.
Now having seen that there are four genera of causes, we have to know that it is not impossible for the same thing to have several causes: like a statue, the causes of which are both the bronze and the sculptor, but the sculptor as efficient, while the bronze as its matter. Neither is it impossible for the same thing to be the cause of contraries. For example, the pilot can be the cause both of the salvation and of the sinking of the ship, but of the one by his presence, while of the other by his absence. We also have to know that it is possible that something be both cause and effect in respect of the same thing, but not in the same way: for walking is the cause of health as its efficient, but health is the cause of walking as its end: for we take a walk sometimes for the sake of our health. Again, the body is the matter of the soul, while the soul is the form of the body. Also, the efficient is said to be the cause of the end, for the end comes to be by the operation of the agent, but the end is the cause of the efficient, insofar as the agent operates only for the sake of the end. Whence the efficient is the cause of the thing that is the end, say, health; but it does not cause the end to be the end; as the doctor causes health, but he does not cause health to be the end. On the other hand, the end is not the cause of the thing that is the efficient, but is the cause for the efficient to be efficient: for health does not cause the doctor to be a doctor (and I am speaking about the health that is produced by the operation of the doctor), but it causes the doctor to be efficient, so the end is the cause of the causality of the efficient, for it causes the efficient to be efficient, and similarly, it causes matter to be matter and form to be form, for matter does not receive form, except by the end, and form does not perfect matter, except by the end. Whence it is said that the end is the cause of all causes, for it is the cause of the causality of all causes. For matter is said to be the cause of form, insofar as the form exists only in matter; and similarly, form is the cause of matter, insofar as matter has actual existence only by the form. For matter and form are correlatives, as is said in the second book of Physics. They are related to the composite substance, however, as parts and as simple to the composite.
But since every cause is naturally prior to its effect, we should know that something is called “prior” in two ways, as Aristotle says in the sixteenth book of his On Animals. And by this difference something can be called both prior and posterior in respect of the same thing, and both cause and effect. For something is said to be prior to something else in respect of generation and time, and again, in respect of substance and completion. Now since the operation of nature proceeds from what is imperfect to what is perfect and from what is incomplete to what is complete, what is imperfect is prior to what is perfect in respect of generation and time, but what is perfect is prior in completion. So we can say that a man is prior to a boy in substance and perfection, but the boy is prior to the man in generation and time. But although among generable things that which is imperfect is prior to what is perfect, and potentiality is prior to act (considering the same thing that is imperfect prior to becoming perfect, and is in potentiality, prior to becoming actual), nevertheless, absolutely speaking, what is actual and perfect is necessarily prior: for what reduces that which is in potentiality to actuality is in actuality, and what perfects the imperfect, is itself perfect. Now matter is prior to form in generation and time: for that to which something is coming is prior to what is coming to it. Form, however, is prior to matter in perfection, since matter has no complete existence, except by the form. Similarly, the efficient is prior to the end in generation and time, for it is from the efficient that motion starts toward the end. But the end is prior to the efficient, insofar as it is efficient, in substance and completion, for the action of the efficient is completed only by the end. So these two causes, namely, matter and the efficient, are prior in generation; but the form and the end are prior in perfection.
And we should note that there are two kinds of necessity: absolute necessity and conditional necessity. Absolute necessity proceeds from those causes that are prior in generation, which are matter and the efficient: for example, the necessity of death derives from matter and the disposition of the contrary components of the body; and this is called absolute, because it cannot be impeded. And this type of necessity is also called the necessity of matter. Conditional necessity, on the other hand, proceeds from those causes that are posterior in generation, namely, form and the end. For example, we say that conception is necessary, if a man is to be generated; and this is conditional, for it is not necessary for this woman to conceive, unless under this condition, namely, that if a man is to be generated. And this is called the necessity of the end.
We should also know that three causes can coincide, namely the form, the end and the efficient, as is clear in the generation of fire. For fire generates fire, so fire is the efficient, insofar as it generates; again, fire is form, insofar as it makes actual that was previously potential, and again, it is the end, insofar as it is intended by the agent, and insofar as the operation of the agent is terminated in it. But there are two kinds of ends, namely the end of generation and the end of the thing generated, as is clear in the generation of a knife. For the form of the knife is the end of its generation; but cutting, which is the operation of the knife, is the end of the thing generated, namely of the knife. Now the end of generation sometimes coincides with two of the above-mentioned causes, when something is generated by something of the same species, as when man generates man, and olive generates olive. But this may not be understood for the end of the thing generated.
We should know, however, that the end coincides with the form numerically, for it is numerically the same item that is the form of the generated thing and that is the end of generation. But with the efficient it does not coincide numerically, but can coincide specifically. For example, when a man generates a man, then the generating man and the generated man are numerically different, but are specifically the same. But matter does not coincide with the others, because matter, since it is a being in potentiality, is by its very nature imperfect, while the other causes, since they are actual, are by their nature actual; but what is perfect and what is imperfect never coincide.
Now having seen that there are four kinds of causes, namely, efficient, material, formal and final, we have to know that any of these kinds is divided in various ways. For some causes are called prior and some are called posterior, as when we say that the art of medicine and the doctor are both causes of health, but the art is the prior, while the doctor is the posterior cause; and similarly in the case of formal and the other kinds of causes.
Note here that in our inquiry we always have to go back to the first cause, as when we ask: Why is he healthy? The answer is: because the doctor cured him. And then, further: How did he cure him? The answer is: by his knowledge of medicine. And we should know that it is the same thing to say that a cause is posterior and that it is proximate, or that a cause is prior and that it is remote. So these two divisions of causes, namely, into prior vs. posterior and into proximate vs. remote, signify the same. But we should know that the more universal cause is always called the remote cause and the more specific cause is called the proximate cause. For example, we say that the proximate form of man is what his definition signifies, namely rational, mortal animal, but his more remote form is animal and the even more remote one is substance. For all superiors are forms of the inferiors. Similarly, the proximate matter of the statue is bronze, while the more remote is metal and the even more remote one is body.
Again, some causes are per se, others are per accidens. A per se cause of a thing is its cause insofar as such, as the builder is the cause of the house, or the wood is the matter of the bench. A cause per accidens is one that coincides with the cause per se, as when we say that the doctor is building. For the doctor is a cause per accidens of the building, because he is building not insofar as a doctor, but insofar as coincides with the builder. And the situation is similar in all other cases.
Again, some causes are simple, some are composite. Something is called a simple cause, when it is named only by the name of the per se cause, or only by the name of the per accidens cause, as when we say that the builder is the cause of the building, and similarly when we say that the doctor is the cause of the building. But a cause is called composite, when we name it by the name of both, as when we say that the builder-doctor is the cause of the building.
But also that can be called a simple cause, according to Avicenna’s exposition, which is a cause without the addition of anything else, as the bronze of the statue, for the statue is made of bronze, without the addition of any other matter; or when we say that the doctor causes health, or the fire causes heat. We have a composite cause, however, when the cooperation of many things is needed for causation; e.g. one man cannot move a ship, but many can, or one stone does not make a house, but many stones do.
Again, some causes are actual, some are potential. An actual cause is one that is actually causing the thing, as when the builder is actually building, or the bronze, while the statue is actually being made of it. A potential cause, on the other hand, is what is not actually causing the thing, but can cause it, like the builder, when he is actually not building. And we should know that the actual cause and its effect should exist at the same time, so that if one of them exists, also the other is necessarily exists. For if the builder is actually working, then he has to be building, and if the act of building actually takes place, it is necessary that the builder be actually working. But this is not necessary in the case of merely potential causes.
We should know further that a universal cause is compared to a universal effect and a singular cause is compared to a singular effect. For example, we say that a builder is the cause of a building in general, but also that this builder is the cause of this building in particular.
We should also know that we can speak about the agreements and differences of the principles in terms of the agreements and differences of what they are the principles of. For some things are numerically identical, as Socrates and this man, pointing to Socrates; some things are numerically different, and specifically the same, as Socrates and Plato, who, although are both humans, are nevertheless numerically distinct. Again, some things differ specifically, but are generically the same, as a man and a donkey both belong to the genus of animals; still others are the same only analogically, as substance and quantity, which do not agree in some genus, but agree only analogically: for they agree only in that they are beings. But being is not a genus, because it is not predicated univocally, but analogically.
To understand this better, we have to know that it is in three ways that something can be predicated of several things: univocally, equivocally and analogically. Something is predicated univocally, if it is predicated by the same name and according to the same concept or definition, as “animal” is predicated of a man and a donkey. For both of them are said to be animals, and both are animated sensible substances, which is the definition of animal. Something is predicated equivocally, if it is predicated of several things by the same name, but according to different concepts, as “dog” is predicated both of the barking animal and of the constellation, which agree only in this name but not in the definition or signification of this name: for what is signified by a name is its definition, as is said in the fourth book of the Metaphysics. Finally, something is predicated analogically, if it is predicated of several things, the concepts of which are different, yet are related to the same thing. For example, “healthy” is said of the body of animals and of urine and of food, but it does not signify the same in all these cases. For it is said of urine, insofar as it is a sign of health, of the body, insofar as it is the subject of health, and of the food, insofar as it is the cause of health; but all of these concepts are related to one and the same end, namely, health. For sometimes those that agree analogically, i.e., proportionally, are related to the same end, as is clear in the above example, but sometimes they are related to the same agent, as when “healer” is said of someone who operates by the art of medicine, as a doctor, or without it, as a midwife, or even of the instruments, but always in relation to the same agent, namely the art of medicine. Again, sometimes they are related to the same subject, as when “being” is predicated of substance, of quality, of quantity and of the other categories. For it is not wholly the same concept according to which a substance is said to be a being, and a quantity and the rest, but all these are said to be beings only in relation to substance, which is the subject of all of them. So “being” is said primarily of substance, and only secondarily of the rest. Whence “being” is not a genus, for no genus is predicated primarily and secondarily of its species, but “being” is predicated analogically. And this is what we said, namely, that substance and quantity differ generically, but they are the same analogically.
Now of those that are numerically the same, also the form and
matter is numerically the same, as Tully’s and
 The contrast in the Latin is that between esse, to be, absolutely speaking, and esse aliquid, literally, to be something. But since Aquinas’ point here is the contrast between the substantial being of a thing on account of which it exists as a substance of some kind and its accidental being on account of which it is in a way, say, as being of such and such a shape, size, color, etc., the idea is better brought out in English by contrasting being absolutely with being somehow.
 The literal rendering of the distinction in Latin (between materia ex qua and in qua, i.e., matter “from which” and “in which” something is made, respectively) would not be as helpful as the existing English distinction between matter that a thing is “made from” and matter that it is “made of”. The former member of the existing distinctions in both languages indicates the transient matter of a thing, that from which it is made through some substantial transformation of this matter. This is how we say that bread is made from flour. But we cannot say that the bread is made of flour. The latter construction indicates the permanent matter of the thing, which is actually present in the constitution of the thing as long as the thing exists. This is how we say that a statue is made of bronze (but, again, a bronze statue is made from tin and copper).
 In its ordinary, common sense, the Latin word pricipium from which the English word ‘principle’ derives simply denotes the beginning or first member of any series of items.