If post-Cartesian philosophy is to be described as busying itself with
the problem of the antithesis of mind and matter, the pantheistic
monism of Spinoza may be designated as an attempt to solve the problem
by merging matter and mind in the unity of the infinite substance, and
the empirical movement as an attempt to eliminate the antagonism by
reducing mind to matter. The idealistic movement, which was represented
by Leibniz and Berkeley, was still another essay to remove the
antithesis between mind and matter, by reducing matter to mind.
Perhaps, however, the true significance of the idealistic movement will
be best understood if it is regarded rather as
an attempt to restore the aesthetic and religious ideals which were
threatened by the first empiricists and destroyed by the atheistic and
materialistic empiricists of later times. But, whether we represent the
idealistic movement as a solution of the Cartesian problem, or as a
reaction against the purely scientific concept of philosophy, it will
be evident, in either case, that Leibniz represents a more hesitating
and less thorough, while Berkeley represents a more pronounced and more
complete, form of idealism.
Life. Gottfried Wilhelm Leibniz was born at Leipzig in 1646. At
the age of fifteen he entered the university of his native city,
devoting himself to the study of law and philosophy. After obtaining
the degree of master of philosophy at Leipzig, and that of doctor of
laws at Altdorf, he went to the court of the elector of Mainz, by whom
he was sent on a diplomatic mission to Louis XIV of France. In France,
England, and Holland he formed the acquaintance of the most learned men
of the time, and, with the ample means at his disposal, he had no
difficulty in acquiring a wonderfully wide and accurate knowledge of
all the scientific and philosophical literature of the day. From 1676
until his death, in 1716, Leibniz resided at Hannover, where he held
the offices of court counselor and librarian.
Sources. Leibniz did not compose a complete and extended
exposition of his philosophy. His writings are, for the most part,
brief treatises and essays on various scientific and philosophical
problems. The most important of these are Disputatio Metathysica de
Principio Individui, La monadologie, Essais de
théodicée, and Nouveaux essais sur l'entendement
humain (reply to Locke's Essay). The principal editions of
his collected works are those of Raspe (Leipzig and Amsterdam, 1765),
Dutens (Geneva, 1768), Erdmann (Berlin, 1840), Foucher de Careil
(Paris, 1859 ff.), and Paul Janet (Paris, 1866). Merx's Leibniz
(Blackwood's Philosophical Classics, Edinburgh and Philadelphia,
1884) and Dewey's Leibniz's New Essays (Chicago, 1888) will be
found useful in the study of Leibniz' philosophy. 
 For summary of Leibniz' doctrines, cf. Brucker in preface to
Duten's edition of Leibniz' works (Leibnitii Opera Omnia,
Geneva, 1768, Vol. I, pp. 143 ff.). Consult article on Leibniz in
Encyc. Brit.; also Thilly, Leibnizens Streit gegen
Locke, Heidelberg, 1891.
General Standpoint. Descartes had started his philosophical
speculations with the desire to isolate himself from his fellow-men,
and to build up a philosophy which should owe nothing to his
predecessors. Leibniz, on the contrary, was inspired with the thought
of founding a system which should reconcile all the systems of his
predecessors, bring Plato into harmony with Democritus, demonstrate
the agreement of Aristotle with Descartes, and prove that there is no
inherent contradiction between Scholasticism and modern thought. 
This was in keeping with the many-sided and cosmopolitan character of
the man who, as discoverer of the differential calculus, ranked among
the foremost mathematicians of his day, and was equally eminent as a
scientist, a philosopher, and a religious controversialist.
 Cf. Fouillée, Histoire de la Philosophie
(Paris, 1891), p. 306.
With a view to effecting this universal harmony of systems, Leibniz
adopted a theory of reality which centers on the doctrine of
monads, the principle of preëstablished harmony, and
the law of continuity. He sought to establish the perfect
correspondence of mind with matter and the participation of matter by
mind and of mind by matter (pan-psychism).
Doctrine of Monads. Leibniz, like Spinoza, considers that the
notion of substance is the starting point in metaphysical speculation.
But, while Spinoza defines substance as independent existence, Leibniz
defines it as independent power of action: "La substance ne saurait
etre sans action."  From this difference there arises another: if
substance be defined as self-existence, it is necessarily one, and
hence Spinoza was consistent with his definition when he taught that
substance is one; whereas, if substance be defined as self-activity, it
is essentially individual, and at the same time necessarily manifold.
The manifold individual substances are monads.
 Cf.Opera, I, 733, and II, P. II, 19.
The monads are analogous to atoms; they are simple, indivisible,
indestructible units.  They differ from the atoms in this, that no
two monads are alike. They differ also in respect to indivisibility;
for the atom is not an absolutely indivisible point, while the monad is
a metaphysical point, real and indivisible. Finally, they differ from
atoms in this, that the atom is merely a material constituent of
bodies, whereas the monad is immaterial, in so far, namely, as it is
endowed with the power of representation.
 Cf.op. cit., II, P. I, 22ff.
This power of representation is the essence, so to speak, of the
monad. Leibniz is careful to distinguish between conscious and
unconscious representation. Some monads, as for instance the human
soul, are conscious of what they represent; others represent
unconsciously; each monad, whether consciously or unconsciously,
reflects every other monad in the universe. Each monad is therefore a
microcosm, a multiplicity in unity, a mirror of all reality, in which
an all-seeing eye might observe what is taking place all over the
world.  One monad differs from another merely in this, that while
both represent all reality, one represents it more perfectly than the
other. Now, since all the activity of the monad consists in
representing, and since there are different degrees in the perfection
with which a monad represents other monads, every monad must be dual,
partly active and partly passive. Retaining the Aristotelian
terminology, while modifying the meaning of the terms, Leibniz calls
the passive element the matter, and the active element the
form, or entelechy, of the monad. God alone represents all
monads with perfect clearness, and is therefore pure actuality; all
other monads represent imperfectly, and are therefore partly active
(clearly representative) and partly passive (confusedly
representative), that is, composed of form and matter. It was thus that
Leibniz strove to reconcile the schoolmen with modern thought.
 Cf. op. cit., II, P. I, 33.
Everything in the universe is composed of monads and everything takes
its place in the scale of perfection according to the degree of
clearness with which it represents other monads. Every monad is partly
material and partly immaterial, so that from the lowest monad, which
represents unconsciously, and shows its unconscious perception in the
phenomena of attraction and repulsion, up to the highest created monad,
which is the human soul, there is absolute continuity without
interruption or unnecessary duplication. This is known as the law of
continuity.  Its counterpart is the law of indiscernibles.
If there is no unnecessary duplication, there is no perfect similarity
of forms, and, indeed, since no two monads represent the universe in
exactly the same manner, no two are perfectly alike. If they were
exactly alike they would not be two, but one; for it is the manner of
representation that constitutes the individuality of a monad.
 Cf.op. cit., I, 366.
Preëstablished Harmony. If each monad is a little universe
in itself, reflecting every other monad, and individuated by its manner
of representing, if it develops this power from the germs of activity
inherent in itself, whence comes the correspondence of one
representation with another, and the resulting harmony of the entire
system of monads? Leibniz answers by postulating a divine
arrangement by virtue of which the monads have from the beginning
been so adapted to one another that the changes of one monad, although
immanent, are parallel to the changes in every other monad of the
cosmic system. This doctrine of preestablished harmony,  which is
germinally contained in Descartes' doctrine of the relations of the
soul to the body, finds its most important application in psychology.
Soul and body have no direct influx on each other, but, just as two
clocks may be so perfectly constructed and so accurately adjusted that
they keep exactly the same time, so it is arranged that the monads of
the body put forth their activity in such a way that
to each physical activity of the monads of the body there
corresponds a psychical activity of the monad of the soul.
 Cf.op. cit., II, P. I, 24, 40, 65.
When we inquire into the ultimate foundation of this harmony, and look
for the reason of the divine arrangement on which the harmony of the
universe depends, we find an answer in Leibniz' optimistic principle,
the lex melioris. Of possible worlds God chose the best, and,
even apart from the divine choice, the best would necessarily prevail
over all other possible worlds, and become actual. This lex
melioris is itself founded on the law of sufficient reason, 
that, namely, things are real when there is sufficient reason for their
existence. The law of sufficient reason is, according to
Leibniz, a law of thought as well as a law of being.
 Cf.op. cit., I, 152.
Psychology. From the definition of the monad it is clear that
all created reality is partly material and partly immaterial, that
there are no bodiless souls and no soulless bodies. Moreover, the law
of continuity demands that the soul always think, that reason and sense
differ merely in degree, and that sense-knowledge precede rational
knowledge. Yet, although the soul, the "queen-monad," is akin to other
monads, and although the law of continuity forbids a gap between the
soul of man and lower forms, the human soul possesses intellectual
knowledge by which it is discriminated from the souls of lower animals.
Whence comes this intellectual knowledge? What is the origin of our
ideas? In the Nouveaux essais, Leibniz not only contradicts
Locke's doctrine that none of our ideas are innate,  but lays down
the contrary proposition and maintains that all our ideas are innate.
He teaches that the soul has "no doors or windows" on the side facing
the external world, that, consequently, all our knowledge is developed
from germs of thought which are innate. The innateness of our ideas is,
however, implicit rather than explicit. Ideas exist potentially in the
mind, so that the acquisition of knowledge is the evolution of
the virtually existent into the actually existent. To the principle,
"Nihil est in intellectu quod prius non fuerit in sensu," Leibniz adds,
"nisi ipse intellectus." 
 Nouveaux essais, Preface.
 Cf.op. cit. II, 1.
Have our ideas, therefore, any objective value? Leibniz answers that
they have, because the evolution of the psychic monad from virtual to
actual knowledge is paralleled by the evolution of the cosmic monad in
the outside world. Here, as elsewhere, the harmony is preëstablished.
The immortality of the soul follows from its nature. The soul is
a monad, self-active, self-sufficient (suffisant à
lui-même), and is therefore as lasting as the universe
Theodicy. Leibniz' principal treatise on natural theology, the
Théodicée,  was composed for the purpose of
refuting Bayle, who had tried to show that reason and faith are
incompatible. The work is devoted, in a large measure, to the
discussion of the problem of evil and to the defense of optimism.
 In Duten's edition, I, 117 ff., Tentamina Theodiceae de
Bonitate Dei Libertate Hominis et Origine Mali.
Leibniz' arguments to prove the existence of God may be reduced
to three: (1) from the idea of God (a modification of Descartes'
proof); (2) from the contingency of finite being; and (3) from the
character of necessity which our ideas possess. Ideas possess not
merely hypothetical but absolute necessity, -- a necessity which cannot
be explained unless we grant that an absolutely necessary Being exists.
When it is said that the idea of God plays a teleological rather than a
scientific rôle in Leibniz' system of thought, the meaning of
this is that Leibniz is interested not so much in giving an account of
the origin of the universe, as in discovering an absolute final cause
towards which all created being tends. Indeed, we find that the
idealist is always more inclined than is the empiricist to fall back on
the teleological explanation, and in the philosophy of Leibniz the
teleological concept is of especial
importance as the foundation of the principle of sufficient reason. It
is also of importance as affording a solution of the problem of
evil, -- a problem to which Leibniz devoted much attention.  He
distinguishes metaphysical evil, which is mere limitation or
finiteness, physical evil, which is suffering, and moral
evil, which is sin. The ultimate source of all evil is the
imperfection which of necessity attaches to limited existence, and
which therefore must be permitted by God, although it is reduced by Him
to the minimum, and made to serve a higher purpose, -- the beauty and
harmony of the whole. Leibniz exhorts us to consider evil, not in its
relation to parts of reality, but in its relation to the totality of
being. "We can see," he writes, "only a very small part of the chain of
things, and that part, moreover, which displays the most evil, and
which is, therefore, well suited to exercise our faith and our love of
 Cf.Opera, I, 478 ff.
 Letter to Bourguet, quoted by Höffding. op. cit., I, 366.
Historical Position. The philosophy of Leibniz cannot, like that
of Locke, be characterized as superficial. It takes up, and attempts to
solve, the most important questions of metaphysics and psychology. In
spirit and tone, rather than in method and content, it is Platonic,
that is, inspired by idealism and inclined to the poetic rather than to
the scientific synthesis. And herein lies its principal defect: it is
unreal. For although Leibniz was as fully alive as was any of his
contemporaries to the importance of scientific study and experimental
investigation, his philosophy is built, not on the data of experience,
but on a priori definitions and principles. Yet we must not on
this account underrate the importance of Leibniz as a speculative
thinker. He rendered inestimable service to the cause of philosophy by
setting himself in determined opposition to the current of empirical
sensism. Besides, the study of his philosophy is healthful; it expands
the mind, opens up new vistas of philosophic syntheses, and is an
invaluable aid to the understanding of subsequent systems.
In the philosophy of Berkeley we find another phase of idealism, an
idealism carried to the point of the absolute denial of the reality of
Life. George Berkeley was born at Dysert, in County Kilkenny,
Ireland, in the year 1685. After having made his elementary studies at
Kilkenny, he went, in 1700, to Trinity College, Dublin, where, owing to
the influence of Molyneux, the philosophy of Locke was in the
ascendency. From the Common-place Book, in which, as early as
1705, Berkeley began to set down his thoughts on philosophical
problems, it appears that, while still at Trinity College, he had begun
to study Descartes and Malebranche as well as Locke. In 1709 he
published his New Theory of Vision, and in the following year,
his Principles of Knowledge. In 1713 he went to London, where he
formed the acquaintance of Steele, Collins, Swift, Pope, and Addison,
and in the winter of the same year he visited Père Malebranche
at Paris. After several years spent in France and Italy, he returned to
London in 1720, to find the whole country in a turmoil over the failure
of the South Sea Scheme. It was this condition of affairs that prompted
Berkeley to write his Essay towards Preventing the ruin of Great
Britain. In 1721 he returned to Ireland to receive a deanery in the
Established Church. From 1723 until 1731 he was occupied with his
famous scheme for converting the American Indians and with the project
of founding for that purpose a college in Bermuda. The two years which
he spent at Whitehall, near Newport, Rhode Island, while waiting for
the government grant promised by Sir Robert Walpole, afforded him an
opportunity to continue his philosophical studies and to make the
acquaintance of Samuel Johnson, through whom he may be said to have
influenced Jonathan Edwards, the first representative of philosophy in
America. On returning to London in 1731, Berkeley published his
Alciphron, or the Minute Philosopher, a dialogue directed
against the freethinkers (minute philosophers). In 1734 he was made
bishop of Cloyne, in Cork. In that "serene corner" he combined the
study of Plato with the advocacy of tar water as a cure for all human
ills, publishing Siris; A Chain of Philosophical Reflections and
Inquiries concerning the Virtues of Tar Water, etc. In 1752 he went
Oxford, where he died in the following year.
Sources. Berkeley's most important works are An Essay toward
a New Theory of Vision, A Treatise concerning the Principles of Human
Knowledge, Three Dialogues between Hylas and Philonous, Alciphron, or
the Minute Philosopher, the Analyst, and Siris. The best
edition of his
collected works is Fraser's (4 vols., Clarendon Press, 1871, new
edition, 1901). Fraser's Berkeley (Blackwood's Philosophical
Classics, Edinburgh and Philadelphia, 1894) is an excellent
introduction to the study of Berkeley and his philosophy. 
 Consult also Fraser's Selections from Berkeley (fourth
edition, London, 1891) and Simon, Universal Immaterialism
General Aim of Berkeley's Philosophy. In the Common-place
Book, of which mention has already been made, we find the following
entry: "The chief thing I do, or pretend to do, is only to remove
the mist and veil of words." The great obstacle to the discovery
and acceptance of truth is, Berkeley thinks, the use of words which
represent abstractions of the mind and prevent us from arriving at a
knowledge of "things." Locke had indeed announced the principle that
our knowledge extends only to ideas; but he straightway proceeded,
Berkeley observes, to violate this very principle when he maintained
that we know the qualities and powers of things outside the mind and
have a "sensitive knowledge of their existence. Berkeley, therefore,
starts where Locke had started, but he aims at going farther than Locke
had gone, -- at establishing the truth of the conclusion that "all
things are ideas," a conclusion which Berkeley regards as necessarily
involved in Locke's principle that our knowledge extends to ideas
Immaterialism. In his New Theory of Vision, Berkeley
takes the first step in the direction of immaterialism. He shows, in
the first place, that the only phenomena which we perceive by means of
sight are colors, and that with these we associate the phenomena of
touch and muscular movement. He then proceeds to show that the reason
of the association is "custom," "experience," or "suggestion." The
conclusion is that what we "see" in the world around us is far more
dependent on mind than we are commonly aware of.
In the Treatise concerning the Principles of Human Knowledge, he
takes up once more the problem of knowledge, and endeavors to show that
what he had proved to be true of the phenomena of sight is true of the
whole phenomenal world of sense; he tries, moreover, to find the reason
for the custom, experience, or suggestion, by virtue of which we
associate certain phenomena with certain others. He teaches that all
the qualities of matter, primary as well as secondary, resolve
themselves into mind-dependent phenomena. What, then, is it that
groups these phenomena, for example, the color, size, shape, etc., of
an orange, into those clusters or aggregates which we call "things"?
The answer that phenomena are grouped together by an inert, lifeless
matter is self-contradictory, because phenomena, being essentially
mind-dependent ideas, cannot exist in an unperceiving substance. 
Besides, matter is a mere abstraction, one of those words which merely
serve to throw a "veil and mist" between the mind and a knowledge of
truth. It is evident, therefore, that both the popular and the
philosophical conceptions of matter are absurd. There is no material
substratum of things; mind and mind-dependent phenomena alone exist; to
be is to be perceived, -- esse est percipi.
 Works, I, 142. References are to Fraser's edition
(Clarendon Press, 1871).
Yet the world is not a chaos, but a cosmos: there is a continual change
and succession of phenomena, and in all this change and succession
there is order and regularity. "There is, therefore, some cause of
these ideas, whereon they depend. . . ,but it has been shown that there
is no corporeal or material substance: it remains, therefore, that the
cause of ideas is an incorporeal substance or spirit."  Now, since
the ideas actually perceived sense have no dependence on my will, it
follows that it is not ny mind but the eternal, uncreated spirit
that produces them. 
 Op. cit., I, p. 169.
 Op. cit., I, p. 172.
Matter does not exist: spirit exists; the "external world" is spirit
and the phenomena which spirit produces in the created
mind; the only noumenal realities are God and human minds, -- these
are the conclusions in which Berkeley's immaterialism is summed up. It
follows that there are no secondary causes and that the laws of nature
are really laws of the Eternal Spirit.
Theism. In the Dialogues, and especially in the
Alciphron, Berkeley undertook to show what is meant by the
Eternal Spirit, to Whom he had, in his earlier treatises, referred the
persistence and activity of the phenomena into which he bad analyzed
the "external world." His line of reasoning may be described as
analogical: just as we "see" men we "see" God. As we argue from
the phenomena of sight, hearing, etc., to the existence of the human
spirit in men, so we may argue from the phenomena of sense in general
to the existence of the Infinite Spirit Whose thoughts (physical laws)
are conveyed to us in the language of sense phenomena (physical
qualities). Alciphron, the sceptic, confesses, "Nothing so much
convinces me of the existence of another person as his speaking to me."
To which Euphranor replies, "You have as much reason to think the
Universal Agent, or God, speaks to your eyes as you can have for
thinking any particular person speaks to your ears." 
 Op. cit., II, 146 ff.
Platonism. The study of Plato, which, during his residence at
Cloyne, Berkeley combined with the study of the medicinal properties of
tar water, developed in the mind of our philosopher a growing
tendency towards a mystic view of the problem of the ultimate
reality of things. In the metaphysical portion of the Siris,
which he published at this time, he occupies himself with the problem
of showing how we may arrive at a higher knowledge of God than that
afforded by sense-phenomena. In his Dialogues he was satisfied
with refuting atheism by showing how God speaks to us in nature, but
now he seeks a higher and deeper knowledge. The study of Plato has led
him to the realization of the "uncertain, ever-fleeting, and changing
nature" of sensible things, and to the consequent depreciation
of sense-knowledge as being "properly no knowledge, but only
opinion."  Therefore he counsels the seeker after truth to cultivate
the use of intellect and reason; to penetrate, by the exercise of these
faculties, to a knowledge of the inner nature of things; and through
rational faith in causality, to realize that "there runs a
chain throughout the whole system of beings," and that, by
ascending from what is lower to what is higher, the mind may reach a
knowledge of the Highest Being. This is a lifelong task. "He that would
make a real progress in knowledge must dedicate his age as well as his
youth, the later growth as well as first fruits, at the altar of
 Op. cit., II, 482.
Historical Position. It was Berkeley's intention to remove "the
mist and veil of words," and then from empirical principles to refute
materialism and atheism. If matter does not exist, there is certainly
no justification for materialism, and if all our ideas are produced in
us by the Eternal Spirit, if every act of knowledge implies the
existence of God, then atheism is undoubtedly irrational and untenable.
Berkeley had not the least suspicion of the facility with which
scepticism would take advantage of his immaterialism to reason away
spirit as he himself had reasoned away material substance. "You see,"
says Philonous at the end of the third dialogue, "the water in yonder
fountain; how it is forced upwards in a round column to a certain
height, at which it breaks and falls back into the basin whence rose;
its ascent as well as descent proceeding from the same uniform law or
principle of gravitation. Just so, the same principles which, at
first view, lead to skepticism, pursued to a certain point, bring men
back to common sense."  However, Berkeley built less wisely than he
knew. He carried the principles of empiricism and idealism to a certain
point, -- it is commonly said that he is to Locke what Spinoza is to
Descartes, -- but at that point they were taken up by Hume and carried
their logical conclusion, namely pan-phenomenalism.