The Socratic doctrine of concepts introduced into philosophy the notion
of the universal. No sooner, however, had Socrates formulated the
doctrine of universal concepts than the Cynics arose denying that
anything exists except the individual. Thus it at once became necessary
to define the true relation between the universal and the individual.
This was the aim of Plato's theory of Ideas, in which the relation was
explained by deriving the individual (in reality and in
knowledge) from the universal.
Aristotle, judging that Plato's explanation was a failure, opened up
the problem once more, and endeavored to solve it by deriving the
universal (in reality and in knowledge) from the individual. The
continuity of philosophic thought is, therefore, to be traced from
Socrates, through Plato, to Aristotle, as if the imperfect Socratic and
Platonic schools had not existed.
Life.  Aristotle was born at Stagira, a seaport town of the colony of
Chalcidice in Macedonia, in the year 384 B.C. His father, Nicomachus,
was physician to King Amyntas of Macedon, and if, as is probable, the
profession of medicine was long hereditary in the family, we may
suppose that this circumstance was not without its influence in
determining Aristotle's predilection for natural science. When he was
eighteen years old, Aristotle went to Athens, where for twenty years he
followed the lectures of Plato. Many stories are told concerning the
strained relations between the aged teacher and his illustrious
scholar, -- stories which, however, are without any foundation.
There may indeed have been differences of opinion between master and
pupil, but there was evidently no open breach of friendship, for in
later years Aristotle continued to count himself among the Platonic
disciples,  associated with Xenocrates on terms of intimate friendship,
and showed in every way that his respect for his teacher was not
lessened by the divergence of their philosophical opinions. Many of
the tales told to Aristotle's discredit are traced to Epicurus and
the Epicureans, -- calumniators by profession (grubbers of gossip, as
Zeller calls them), -- and it is to be regretted that writers like St.
Gregory Nazianzen and Justin Martyr were misled by statements which
were manifestly made with hostile purpose. We are safe, therefore, in
supposing that Aristotle was diligent and attentive pupil, and that he
did not give expression to his criticism of Plato's theories until
after he had listened to everything that Plato had to say in
explanation and defense of his views.
 For sources, cf. Zeller, Aristotle and the Earlier
Peripatetics, Vol. I, p. 2, n.; Wallace, Outlines of the
Philosophy of Aristotle, p. 17.
 As in Met. I, 9, 992 a.
After Plato's death Aristotle repaired, in company with Xenocrates, to
the court of Hermias, lord of Atarneus, whose sister or niece, Pythias,
he married. In 343 he was summoned by Philip of Macedon to become the
tutor of Alexander, who was then in his thirteenth year. The influence
which he exercised on the mind of the future conqueror is described in
Plutarch's Alexander. When Alexander departed on his Asiatic
Aristotle returned to Athens. This was about the year 335. It is
possible that, as Gellius  says, Aristotle had, during his former
residence at Athens, given lessons in rhetoric; it is certain that now
for the first time he opened a school of philosophy. He taught in a
gymnasium called the Lyceum, discoursing with his favorite
pupils while strolling up and down the shaded walks around the
gymnasium of Apollo, -- whence the name Peripatetics (from
 Noct. Att., XX.
 On the derivation of this word, cf. Zeller, op. cit.,
p. 27, n.
Through the generosity of his royal pupil, Aristotle was enabled to
purchase a large collection of books, and to pursue his investigations
of nature under the most favorable circumstances. His writings prove
how fully he availed himself of these advantages: he became thoroughly
acquainted with the speculations of his predecessors and neglected no
opportunity of conducting, either personally or through the
observations of others, a systematic study of natural phenomena.
Towards the end of Alexander's life the relations between the
philosopher and the great commander became somewhat strained. Still, so
completely was Aristotle identified in the minds of the Athenians with
the Macedonian party that after Alexander's death he was obliged to
flee from Athens. The charge which was made the pretense of his
expulsion from the city was the stereotyped one of impiety, to which
charge Aristotle disdained to answer, saying (as the tradition is) that
he would not give the Athenians an opportunity of offending a second
time against philosophy. Accordingly, he left the city (in 323),
repairing to Chalcis in Euboea. There he died in the year 322, a few
months before the death of Demosthenes. There is absolutely no
foundation for the fables narrated by so many ancient writers and
copied by some of the early Fathers, that he died by poison or that he
committed suicide by throwing himself into the Euboean Sea "because he
could not explain the tides."
Aristotle's Character. Eusebius, in his Praeparatio
Evangelica, XV, 2, enumerates and refutes the accusations which
were brought against Aristotle's personal character, quoting from
Aristocles, a Peripatetic of the first century B.C. These accusations
are practically the same as those which gained currency among the
enemies and detractors of Plato, and are equally devoid of foundation.
From Aristotle's writings, from fragments of his letters, from his
will, as well as from the reliable accounts of his life, we are enabled
to form a tolerably complete picture of his personal character. Noble,
high-minded, thoroughly earnest, devoted to truth, courteous to his
opponents, faithful to his friends, kind towards his slaves, he did not
fall far short of the ideal moral life which he sketched in his ethical
treatise. Compared with Plato, he exhibited greater universality of
taste; he was not an Athenian; in a certain sense, he was not a Greek
at all. He exhibited in his character some of that cosmopolitanism
which afterwards became a trait of the ideal philosopher.
Aristotle's Writings. It is quite beyond dispute that some of
the works which Aristotle compiled or composed have been lost. Thus,
for example, the anatomai (containing anatomical charts), the
peri phitôn (the existing treatise De Plantis is by
Theophrastus), the politeiai (a collection of constitutions of
states; the portion which treats of the Constitution of Athens has been
discovered in recent years), and the Dialogues are among the
lost works. It is equally certain that many portions of the collected
works of Aristotle as we now possess them are of doubtful authenticity,
while it is possible that a still larger number of books or portions of
books are little more than lecture notes amplified by the pupils who
edited them. It is well, for example, for the student of the
Metaphysics to know that, of the fourteen books which compose
it, the first, third, fourth, sixth, seventh, eighth, and ninth
constitute the work as begun but not finished by Aristotle. Of the
remaining books, the second and one half of the eleventh are pronounced
spurious, while the rest are independent treatises which were not
intended to form part of the work on first philosophy. Without entering
into the more minute questions of authenticity, we may accept the
following arrangement of Aristotle's works, with their Latin titles. 
 Cf. Wallace, op. cit., pp. 18 ff.
 Recent editions of Aristotle's Works: the Berlin edition (5 vols.,
1831-1870), which is made the basis of citations; the Didot edition (5
vols., Paris, 1848-1870). For list of translations and secondary
sources, cf. Weber, op. cit., p. 104, n., and Ueberweg,
op. cit., I, pp. 140 and 152.
Constituting the Organon: (1) Categoriae, (2) De
Interpretatione, (3) Analytica Priora, (4) Analytica
Posteriora, (5) Topica, (6) De Sophistices Elenchis.
These were first included under the title of Organon in
The work entitled meta ta phusika (or at least a portion of it)
was styled by Aristotle prôtę philosophia. Its
present title is probably due to the place which it occupied (after
the physical treatises) in the collection edited by Andronicus of Rhodes
(about 70 B.C.).
(1) Physica Auscultatio, or Physica, (2) De Caelo,
(3) De Generatione et Corruptione, (4) Meteorologica, (5)
Historiae Animalium, (6) De Generatione Animalium, (7)
De Partibus Animalium.
(1) De Anima, (2) De Sensu et Sensibili, (3) De
Memoria et Reminiscentia, (4) De Vita et Morte, (5) De
Longitudine et Brevitate Vitae, and other minor works.
(1) Ethica Nicomachea, (2) Politica. The Eudemian Ethics
is the work of Eudemus, although it is probable that it was intended as
a recension of an Aristotelian treatise.
RHETORICAL AND POETICAL TREATISES
(1) De Poetica, (2) De Rhetorica. These are spurious in parts.
Gellius  speaks of a twofold class of Aristotelian writings, the
exoteric, which were intended for the general public,  and the
acroatic, which were intended for those only who were versed in
the phraseology and modes of thought of the school. All the extant
works belong to the latter class. The story of the fate of Aristotle's
works as narrated by Strabo  and repeated with the addition of a few
details by Plutarch,  is regarded as reliable. It tells how the
library of Aristotle fell into the hands of Theophrastus, by whom it
was bequeathed to Neleus of Scepsis. After the death of Neleus the
manuscripts were hidden in a cellar, where they remained for almost two
centuries. When Athens was captured by the Romans in 84 B.C., the
library was carried to Rome by Sulla. At Rome a grammarian named
Tyrannion secured several copies, thus enabling Andronicus of Rhodes to
collect the treatises and publish them. It must not, however, be
inferred that the manuscripts hidden in the cellar for two hundred
years were the only existing copy of Aristotle's works, or that during
all those years the Peripatetic philosophers were without a copy of the
works of Aristotle.
The subsequent history of the Corpus Aristotelicum and the
story of the Syriac, Arabian, and Latin translations belong to the
history of mediaeval philosophy.
 Noct. Alt., XX, 5.
 It is these that Cicero had in mind when he alluded to "the golden
stream of Aristotle's eloquence" (Top., 1,3).
 XIII, 1,54.
 Sulla, 26.
General Character and Division. Aristotle's concept of
philosophy agrees, in the main, with that of Plato. Philosophy is
the science of the universal essence o that which is actual. 
Aristotle is, however, more inclined than Plato was to attach a
theoretical value to philosophy. The difference between the two
philosophers is still greater in their respective notions of
philosophic method. Aristotle does not begin with the universal and
reason down to the particular; on the contrary, he Starts with
particular data of experience and reasons up to the universal essence.
His method is inductive as well as deductive. Consequently, he is more
consistent than Plato in including the natural sciences in philosophy
and considering them part of the body of philosophic doctrine. In fact,
Aristotle makes philosophy to be coextensive with scientific knowledge.
"All science (dianoia) is either practical, poietical, or
theoretical."  By practical science he means politics and
ethics; under the head poietic (poiętike) he includes not
only the philosophy of poetry but the knowledge of the other imitative
arts, while by theoretical philosophy he understands Physics,
Mathematics, and Metaphysics. Metaphysics is philosophy in
the stricter sense the word: it is the knowledge of immaterial Being
or of Being in the highest degree of abstraction (peri
chôrista kai akinęta); it is the pinnacle of all
knowledge, the theological science. In this classification logic
has no place, being apparently regarded as a science preparatory to
 Met., VI, 1, 1028.
 Met., VI, 1, 1025 b, 25.
Our study of Aristotle's philosophy will, therefore,
include: (A) logic; (B) theoretical philosophy, including
(b) physics. (c) mathematics; (C) practical
Philosophy; (D) poietical philosophy.
A. Logic, including Theory of Knowledge. Aristotle does not
employ the word logic in the modern meaning of the term. The
science which we call logic, and of which he is rightly considered the
founder, was known to him as analytic. The Organon, as
the body of logical doctrine was styled by the later Peripatetics,
consists of six parts, or treatises
1. The Categoriae. In the first of his logical treatises
Aristotle gives his classification, or enumeration, of the highest
classes (categories) into which all concepts, and consequently all real
things, are divided; they are substance, quantity, quality,
relation, action, passion, place, time, situation, and
habitus. He intimates that these are intended as classes of
things expressed by isolated words, ta aneu sumplokęs
legomena, that is to say, by words which do not form part of a
proposition. They are to be distinguished, therefore, from the
predicables, or classes of the possible relations in which the
predicate of a proposition may stand to the subject. The predicables
are definition (hopos), genus, difference,
property, and accident.
There can be no reasonable doubt as to the originality of the
Aristotelian arrangement of categories. It is true that there is a
remote analogy between the categories and the distinctions of the
grammarian; but the analogy can be explained without supposing that
Aristotle expressly intended to conform his categories to the
grammatical divisions of words. It is also true that Aristotle does not
always enumerate the categories in the same manner. 
 Cf.Met., VI, 2, 1026 a, 36; V, 8, 1017 a, 24;
Phys., V, I, 223 b, 5.
2. The De Interpretatione. In the second of the logical
treatises, Aristotle takes up the study of the proposition and
the judgment. He distinguishes the different kinds of
propositions, and treats of their opposition and conversion. This
portion of his work forms the core of modern logical teaching.
3. The Analytica Priora contains the treatise on reasoning,
deductive and inductive. In his doctrine of the syllogism
Aristotle admits only three figures. (The syllogism, he teaches, is
based on the Law of Contradiction and the Law of Excluded
Middle.) He mentions three rules of the syllogism. Induction
(epagôgę) he defines as reasoning from the
particular to the general, and though the syllogism, which proceeds
from the general to the particular, is more cogent, in itself,
induction is, for us, easier to understand. The only kind of induction
admitted by Aristotle is complete induction.
4. In the Analytica Posteriora Aristotle takes up the study of
demonstration (apodeixis). True demonstration, as indeed all
true scientific knowledge, deals with the universal and necessary
causes of things. Consequently, all true demonstration consists in
showing causes, and the middle term in a demonstration must, therefore,
express a cause. Not all truths, however, are capable of demonstration.
The first principles of a science cannot be demonstrated in that
science, and principles which are first, absolutely, are indemonstrable:
they belong not to reason, but to intellect (Nous). To the class
of indemonstrable truths belong also truths of immediate
 Cf. Stöckl, op. cit., I, 115; English trans.,
5. The Topica has for subject-matter the dialectical or
problematic syllogism, which differs from demonstration in this,
that its conclusions are not certain but merely probable; they belong
to opinion rather than to scientific knowledge. The Topica also
treats of the predicables.
6. The treatise De Sophisticis Elenchis contains Aristotle's
atudy of fallacies, or sophisms. It contains also an
attack on the Sophists and their methods. Before we proceed to explain
Aristotle's metaphysical doctrines it is necessary to take up the
principles of his theory of knowledge as we find them in the
Analytica Posteriora and elsewhere in his logical and
Theory of Knowledge. Nowhere does the contrast between the
philosophy of Plato and that of Aristotle appear so clearly as in their
theories of knowledge.
1. Plato makes experience to be merely the occasion of scientific
knowledge. Aristotle regards experience as the true source and true
cause of all our knowledge intellectual as well as sensible. 
 Cf.Anal. Post., II, 19, 99 b.
2. Plato begins with the universal (Idea) and attempts to descend to
the particular (Phenomenon); Aristotle, while he recognizes that there
is no science of the individual as such (hę d'
epistęmę tôn katholou),  maintains,
nevertheless, that our knowledge of the individual precedes our
knowledge of the universal: ek tôn kath hekasta gar tou
 Met., XIII, 10, 1086 b, 33.
 Eth. Nic., VI, ii, 1143 b, 5.
3. Plato hypostatized the universal, attributing to it a
separate existence. This, according to Aristotle, is to reduce the
universal to a useless form; for, if the universal exists apart from
the individual, there can be no transition from a knowledge of the one
to a knowledge of the other. The universal, Aristotle teaches,
is not apart from individual things. 
 In Anal. Post., I, 11 init., Aristotle substitutes
the phrase hen kata pollôn for the Platonic hen para ta
4. Finally, according to Plato, the universal, as it exists apart from
phenomena, is a full-blown univeral, endowed with the formal character
of universality; according to Aristotle, the formal aspect of
universality is conferred by the mind, and, therefore, the
universal, as such, does not exist in individual things, but in
the mind alone. This is the only intelligible interpretation of
such passages as Metaphysics, III, 4, 999 and De Anima,
II, 5, 417, in which Aristotle maintains that the individual alone
exists and that the universal is somehow (pôs) in the
 Cf.Met., I, 9, 991 a, 12, 991 b, 1; XIII, 9, 1085
a, 23, etc.; Prantl, Gesch. des. Logik, I, 210 ff.
Aristotle's theory of knowledge, as is evident from the four principles
just explained, recognizes two fundamental attributes of intellectual
knowledge: its essential dependence on sense-knowledge and its
equally essential superiority to sense-knowledge. Aristotle is
as careful to avoid sensism on the one hand as he is to escape idealism
on the other; for, though he admits that all knowledge begins with
experience, he contends that intellectual thought (noęsis)
is concerned with the universal, or intelligible (noęton),
while sense-knowledge has for its object the individual, the
sense-perceived (aisthęton). The distinction of objects
is made the basis and ground of a distinction of faculties and of kinds
of knowledge. 
 Cf.De An., II, 4, 415 b.
If, then, there is a distinction between sense-knowledge and thought,
and if all knowledge begins with sense-knowledge, how do we rise
from the region of sense to that of intellect? Aristotle answers by
distinguishing first and second substance. The first substance
(ousia prôtę) is the individual, which can neither
exist in another nor be predicated of another. Second substance
is the universal, which, as such, does not exist in another, but may be
predicated of another. In the individual substance we distinguish, on
closer examination, two elements, the hupokeimenon or
undetermined, determinable substratum, the matter
(hulę), and the determining principle, or form
(eidos), by which the substance is made to be what it is.  The
essential nature, therefore, the unalterable essence corresponding to
the conncept -- the object, consequently, of intellectual knowledge --
is the form. Matter, it is true, is part of the essential nature, 
but it is, as it were, the constant factor, always the same, and of
itself undifferentiated; it enters into a definition as materia
communis, and when we designate the form of an object, implying the
presence of matter in its general concept, we have answered the question,
What is that object? The form,
then, considered apart from the matter, is the essence of the object
as far as intellectual knowledge is concerned; for intellectual
knowledge has for its object the universal, and since matter is the
principle of individuation, and form the principle of specification,
the conclusion of the inquiry as to the object of intellectual
knowledge is that matter and the individual qualities arising from
matter belong to sense-knowledge, while the form alone, which is the
universal, belongs to intellectual knowledge.  Returning now to the
question, How do we rise from the region of sense to the region of
intellect? the object of sense-knowledge, we repeat, is the whole, the
concrete individual substance. Thought, penetrating through the sense
qualities, reaches the form, or quiddity, lying at the core of the
substance, and this form, considered apart from the material conditions
in which it is immersed, is the proper object of intellectual
knowledge. Thus, the acquisition of scientific knowledge is a true
development of sense-knowledge into intellectual knowledge, if
by development is understood the process by which, under the agency of
the intellect, the potentially intelligible elements of sense-knowledge
are brought out into actual intelligibility. Aristotle himself
describes the process as one of induction
(epagôgę) or abstraction (aphairesis). 
 Cf.Met.. VIII, 6, 1045 a, 12; X, 1, 1052 a, 22.
 Cf.infra, p. 138.
 Phys., 1, 5, 189 a, 7.
 Cf.Anal. Post., I, 13, 81.
B. Theoretical Philosophy, a. Metaphysics. In the
foregoing account of Aristotle's theory of knowledge it has been found
necessary to mention form, matter, and substance, notions which
properly belong to this division of his philosophy.
I. Definition of metaphysics. Metaphysics, or first philosophy,
is the science of Being as Being.  Other sciences have to do
with the proximate causes and principles of Being, and, therefore, with
Being in its lower determinations Metaphysics considers Being as such,
in its highest or most general determinations, and consequently it is
concerned with the highest,
or ultimate, causes. Accordingly, on metaphysics devolves the task of
considering the axioms of all sciences in so far as these axioms are
laws of all existence. For this reason it is that in the
Metaphysics Aristotle takes up the explanation and defense of
the Law of Contradiction.
 Met., IV, 1, 1003 a, 21.
2. Negative teaching. Before proceeding to answer the problem of
metaphysics, What are the principles of Being? Aristotle passes in
review the answers given by his predecessors. He not only recounts the
doctrines and opinions of the pre-Socratic philosophers, -- thereby
adding to his many titles that of Founder of the History of
Philosophy, -- but he also points out what seem to him to be the
shortcomings and imperfections of each school or system. His
criticism of Plato's theory of Ideas is deserving of careful
study, because it is an unprejudiced examination of a great system of
thought by one who was unusually well equipped for the task, and also
because it is the most natural and intelligible introduction to the
positive portion of Aristotle's Metaphysics in which he expounds
his own views.
Both Plato and Aristotle maintain that scientific knowledge is
concerned with the universal (compare Socratic doctrine of concepts).
They agree in teaching that the world of sense is subject to change
and that we must go beyond it to find the world of ideas. Here,
however, they part company. Plato places the world of Ideas, the region of
scientific knowledge, outside phenomena; Aristotle places it in
the sensible objects themselves. it is, therefore, against the doctrine
of a separate world of Ideas that all Aristotle's criticism of Plato's
theory is directed.
(a) In the first place, Aristotle contends  that the Platonic theory
of Ideas is wholly barren. The Ideas were intended to explain
how things came to be and how they came to be known; but they cannot be
principles of Being, since they are not existent in things, and
they cannot be principles of knowledge, since
they exist apart from and have no intelligible relation to the things
to be known. To suppose that we know things better by adding to the
world of our experience the world of Ideas, is as absurd as to imagine
that we can count better by multiplying the numbers to be counted. In a
word, the Ideas are a meaningless duplication of sensible objects.
 Met., I, 9, 991 b.
(b) In the next place, Aristotle  recognizes in the theory of Ideas
an attempt at solving the problem of motion and change. Indeed,
since the Ideas are the only reality, they must contain the principle
of change, for change is a reality; but Plato, by separating the Ideas
from the world of phenomena, and by insisting on the static rather than
on the dynamic phase of the Ideas, precluded all possibility of
accounting for change by means of the Ideas. 
 Cf.Met., I, 9, 991 b.
 Cf. Wallace, op. cit., p. 64.
(c) Moreover, Aristotle finds several contradictions in the Platonic
theory. He is not satisfied with the Platonic doctrine of community
between the Idea and the phenomenon; for, if the participation of the
Idea by the phenomenon is anything more than a mere figure of speech,
if there is really part of the Idea in the phenomenon, there must be a
prototype on which this participation is modeled. If such a prototype
exists, there is, for example, a tritos anthrôpos in addition to
the absolute Idea of man and the man who exists in the world of
phenomena. The significant fact is that Plato at one time describes the
participation as methexis at another as mimęsis and
ends by leaving it unexplained. 
 Cf.Met., XIII, 5, 1079 a, 13.
(d) Finally, the reason why Plato introduced the doctrine of Ideas was
because scientific knowledge must have for its object something other
than the phenomenon. Now, scientific knowledge has an object, if Ideas
exist. The validity of scientific knowledge does not
require that the Idea should exist apart from the phenomenon
 Op. cit., I, 9, 999 a, 12; VII, 1031 a, 20ff.
3. Positive teaching. Metaphysics, as has been said, is the
inquiry into the highest principles of Being. A principle
(archę) is that by which a thing is or is known.  The
first problem of metaphysics is, therefore, to determine the relation
between actuality and potentiality, the first principles
of Being in the order of determination, or differentiation. Actuality
(entelecheia energeia) is perfection, potentiality
(dunamis) is the capability of perfection. The former is the
determining principle of being, the latter is of itself indeterminate.
Actuality and potentiality are above all categories; they are found in
all beings with the exception of One, whose being is all actuality. In
created being, then, as we should say, there is a mixture of potency
and actuality. This mixture is, so to speak, the highest metaphysical
formula, under which are included the compositions of matter and form,
substance and accident, the soul and its faculties, active and passive
intellect, etc. The dualism of actual and potential pervades the
metaphysics, physics, psychology, and even the logic of Aristotle.
 Met., V, 1, 1013 a, 18.
Still, potency and actuality are principles of Being in its
metaphysical determinations. In the physical order, there enter into
the constitution of concrete being four other principles called
causes (aitiai). A cause is defined as that which in
any way influences the production of something: it is, therefore, a
principle in the order of physical determination. The classes of causes
are_four, -- matter (hulę), form
(eidos or morphę), efficient cause (to
kinętikon), and final cause (to hou heneka).  Of
these, matter and form are intrinsic constituents of being, while
efficient and final causes are extrinsic principles. Nevertheless,
these latter are true causes inasmuch as the effect depends on them.
 Cf.Phys., II, 3, 194 b, 16.
Matter, or material cause, is that out of which
being is made; bronze, for example, is the material cause of the
statue. Matter is the substratum (hupokeimenon), indeterminate
but capable of determination. It is the receptacle (dektikon) of
decay.  It can neither exist nor be known without form. In a word,
it is potency. Matter in the condition of absolute potentiality is
called first matter (hule prôtę), that is,
matter without any form. Second matter is matter in the
condition of relative potentiality. Second matter possesses a form, but
because of its capability of further determination it is in potency to
receive other forms.
 De Gen. et Corr., I, 4, 320 a, 2.
Form, or formal cause, is that into which a thing is
made. It is the principle of determination overcoming the
indeterminateness of matter. Without it matter cannot exist: it is
actuality. The Aristotelian notion of form, like the Platonic notion of
Idea, was intended as a protest against the scepticism of the Sophists
and the panmetabolism of the Heracliteans. Form is the object of
intellectual knowledge, the unalterable essence of things, which
remains unchanged amid the fluctuations of accidental qualities. Like
the Idea, the form is the plentitude of actual being, for while matter
is a reality, it is real merely as a potency. There is, however, a
radical difference between the form and the Idea; the form exists in
individual beings, the Idea exists apart from them: Aristotle merely
distinguished matter and form; Plato not only
distinguished but also separated the Idea from the
The union of matter and form constitutes the individual or concrete,
substance (to sunolon, ousia prôtę). From matter
arise the imperfections, limitations, and individuating qualities; from
form come the essential, unalterable attributes, the specific nature of
the substance. Matter, then, being presupposed as the common substratum
of material existence, a substance is constituted in its essential
nature by the form. Hence it is that Aristotle identifies the form with
the essence, the quiddity (to ti en einai),  the universal
nature of a substance. Form is a second substance (ousia deutera) which, while it cannot
inhere in another as in a subject, may, on account of its universality,
be predicated of many. It would, however, be a serious mistake to
represent Aristotle as reducing all reality to form, and ending as
Plato had begun, with the doctrine of monism. For matter, in its
generic concept, enters into the definition of the specific nature, and
while it is not an actual, it is a real principle of being. 
 For the origin and meaning of this expression, cf. Ritter
and Preller, op. cit., p. 314, note f; also Ueberweg,
op. cit., Vol. I, p. 161.
 Cf.Met., VII, 7, 1032 b; VIII, 6, 1045 a, 33; X, I,
1052 a, 22.
Aristotle further develops his theory of the relation between matter
and form by teaching that matter is destined to receive form. It tends
towards its form with something akin to desire: for the absence of form
is not mere negation; it is privation (ateręsis).
Aristotle, however, explains  that matter is not pure privation. It
is a positive something which, of its nature, is disposed to become
determined by means of form.
 Phys., I, 7, 191 a, 10.
Efficient cause is the third principle of being. It is defined
as that by which (that is, by the agency of which) the effect is
produced. Ultimately, it is form considered as operative, for no agent
can act except by virtue of the form, which is the principle of its
action as well as of its being.  Hence the Scholastic adage, Agere
sequitur esse. Moreover, all action is motion (kinęsis), and
motion is defined as the passing from potency to actuality: he tou
duname ontos entelecheia he toiouton.  This identification of
action with motion, and the definition of motion in terms of the actual
and potential, lead at once to a conclusion which is, at first sight,
startling in its universality, -- that all natural processes are
processes of development, and that action merely brings out latent
possibilities by bringing into actuality those perfections which were
already contained as potencies in the matter. This generalization, it
may be remarked, is in perfect harmony with modern physical principles,
as, for example, with the Law of the Conservation of Energy. Aristotle, it is
true, does not enter into the question of quantitative relations
between the potential and the actual. But the higher the human mind
rises in its inquiries, the less attention it pays to questions of
quantitative equivalence, and the more importance it attaches to the
general notion of internal development.
 Cf.op. cit., II, 7, 198 a, 24.
 Op. cit., III, 1, 201 a, 10.
Final cause, the fourth principle of being, that on account of
which the effect is produced, is, in a certain sense, the most
important of all the causes.  It not only determines whether the agent
shall act, but it also determines the mode and manner of the action and
the measure of the effect produced, so that if we could know the motive
or end of an action, we should be in possession of a most fruitful
source of knowledge concerning the result of that action. The final
cause, like the efficient, is, in ultimate analysis, identical with
form; it is the form of the effect, presented in intention and
considered as a motive, inasmuch as by its desirability it impels the
agent to act.
 De Partibus Animalium, I, 1, 639 b, 11.
By the reduction of efficient and final causes to formal cause the
ultimate principles of (finite) being are reduced to two, matter and
form. These are the two intrinsic, essential constituents of the
individual, concrete object, matter being the source of
indeterminateness, potency, and imperfection, while form is the source
of specific determination, actuality, and perfection.
The Aristotelian doctrine of causes is a synthesis of all preceding
systems of philosophy. The Earlier Ionians spoke generically of cause;
the Later Ionians distinguished material and efficient causes;
Socrates, developing the doctrine of Anaxagoras, introduced the notion
of final cause; Plato was the first to speak of formal causes --
unless the Pythagorean notion of number may be regarded as an attempt
to find a formal principle of being. Thus did the generic notion of
cause gradually undergo differentiation into the four kinds of cause.
Aristotle was the first to advert to this historical dialectic of the idea of cause,
and to give the different kinds of cause a place in his doctrine of the
principles of being. Consequently, the Aristotelian doctrine of cause
is a true development, a transition from the undifferentiated to the
differentiated, and nowhere do we realize more clearly than in this
doctrine of cause that Aristotle's philosophy is the culmination of all
the philosophies which preceded it.
According to Aristotle, metaphysics is rightly called the
theological science, because God is the highest object of
metaphysical inquiry. For, although we may in our analysis of the
principles of being descend to the lowest determination, -- or, rather,
to the lack of all determination, -- materia prima, we may turn
in the opposite direction, and by following the ascending scale of
differentiation arrive at the notion of pure actuality, or Being in the
highest grade of determinateness. Aristotle, in his proofs of the
existence of God, did not set aside the teleological argument of
Socrates.  Devoted as he was to the investigation of nature, and
especially to the study of living organisms, he could not fail to be
struck by the adaptation everywhere manifest in natural phenomena, and
particularly in the phenomena of life. He recognized, however, that the
teleological is not the strongest argument for the existence of a
Supreme Being. Accordingly, we find him establishing the existence of
God by means of proofs more properly metaphysical than was the argument
from design. He argues, for example,  that, although motion is
eternal, there cannot be an infinite series of movers and moved; there
must, therefore, be one, the first in the series, which is unmoved, the
prôton kinoun akinęton. Again,  he argues that
the actual is, of its nature, antecedent to the potential.
Consequently, before all matter, and before all composition of actual
and potential, pure actuality must have existed. Actuality is, therefore, the cause of all things
that are, and, since it is pure actuality, its life is essentially free
from all material conditions; it is the thought of thought (noęsis
 Cf.Phys., VIII, I, 252 a.
 Cf.op. cit., VIII, 5, 2562..
 Cf.Met., XII, 6, 1071 b, 20.
To the question, What does Aristotle understand by the primum movens
immobile and the actus purus? the answer seems to be that by
the former of these expressions he meant something other than the
Supreme Being. In the Physics, where he speaks of primum
mobile, or rather of the prima moventia non mota,  he
describes the first being as the first in the order of efficient
causes, an intelligence, the primum coelum. This, which is moved
by the sight of the supreme intelligence of God, not, therefore, by any
efficient cause, but by a final cause only, sets in motion the whole
machinery of efficient causes beneath it. In the Metaphysics,
however, our philosopher pursues his investigation into the realms
beyond the first heaven, and finds that the intelligence which moves by
its desirability the soul of the first heaven is the intelligence of
intelligence, pure actuality, God.  This is the interpretation of St.
Thomas,  who, while he regards God as the immediate efficient cause of
the first motion of the universe, interprets Aristotle to mean that the
First Intelligence moves merely by the desire which He inspires,
drawing towards Him the soul of the first heaven. And it is natural to
expect that in the philosophy of Aristotle there should be a supreme in
the physical order as well as a supreme in the metaphysical order; that
the metaphysical concept of First Intelligence should complete and
round out the physical concept of a first mover. 
 Phys., VIII, 6, 258 b, 12.
 Met., XII, 7, 1072.
 Cf.In XIIum Met. lect.
 Cf. De Vorges in Revue Néoscholastique, 1894,
pp. 304 ff.