Plato's doctrine of ideas has attained an imaginary clearness and definiteness which is not to be found in his own writings. The popular account of them is partly derived from one or two passages in his Dialogues interpreted without regard to their poetical environment. It is due also to the misunderstanding of him by the Aristotelian school; and the erroneous notion has been further narrowed and has become fixed by the realism of the schoolmen. This popular view of the Platonic ideas may be summed up in some such formula as the following: 'Truth consists not in particulars, but in universals, which have a place in the mind of God, or in some far-off heaven. These were revealed to men in a former state of existence, and are recovered by reminiscence (anamnesis) or association from sensible things. The sensible things are not realities, but shadows only, in relation to the truth.' These unmeaning propositions are hardly suspected to be a caricature of a great theory of knowledge, which Plato in various ways and under many figures of speech is seeking to unfold. Poetry has been converted into dogma; and it is not remarked that the Platonic ideas are to be found only in about a third of Plato's writings and are not confined to him. The forms which they assume are numerous, and if taken literally, inconsistent with one another. At one time we are in the clouds of mythology, at another among the abstractions of mathematics or metaphysics; we pass imperceptibly from one to the other. Reason and fancy are mingled in the same passage. The ideas are sometimes described as many, coextensive with the universals of sense and also with the first principles of ethics; or again they are absorbed into the single idea of good, and subordinated to it. They are not more certain than facts, but they are equally certain (Phaedo). They are both personal and impersonal. They are abstract terms: they are also the causes of things; and they are even transformed into the demons or spirits by whose help God made the world. And the idea of good (Republic) may without violence be converted into the Supreme Being, who 'because He was good' created all things (Tim.).

It would be a mistake to try and reconcile these differing modes of thought. They are not to be regarded seriously as having a distinct meaning. They are parables, prophecies, myths, symbols, revelations, aspirations after an unknown world. They derive their origin from a deep religious and contemplative feeling, and also from an observation of curious mental phenomena. They gather up the elements of the previous philosophies, which they put together in a new form. Their great diversity shows the tentative character of early endeavours to think. They have not yet settled down into a single system. Plato uses them, though he also criticises them; he acknowledges that both he and others are always talking about them, especially about the Idea of Good; and that they are not peculiar to himself (Phaedo; Republic; Soph.). But in his later writings he seems to have laid aside the old forms of them. As he proceeds he makes for himself new modes of expression more akin to the Aristotelian logic.

Yet amid all these varieties and incongruities, there is a common meaning or spirit which pervades his writings, both those in which he treats of the ideas and those in which he is silent about them. This is the spirit of idealism, which in the history of philosophy has had many names and taken many forms, and has in a measure influenced those who seemed to be most averse to it. It has often been charged with inconsistency and fancifulness, and yet has had an elevating effect on human nature, and has exercised a wonderful charm and interest over a few spirits who have been lost in the thought of it. It has been banished again and again, but has always returned. It has attempted to leave the earth and soar heavenwards, but soon has found that only in experience could any solid foundation of knowledge be laid. It has degenerated into pantheism, but has again emerged. No other knowledge has given an equal stimulus to the mind. It is the science of sciences, which are also ideas, and under either aspect require to be defined. They can only be thought of in due proportion when conceived in relation to one another. They are the glasses through which the kingdoms of science are seen, but at a distance. All the greatest minds, except when living in an age of reaction against them, have unconsciously fallen under their power.

The account of the Platonic ideas in the Meno is the simplest and clearest, and we shall best illustrate their nature by giving this first and then comparing the manner in which they are described elsewhere, e.g. in the Phaedrus, Phaedo, Republic; to which may be added the criticism of them in the Parmenides, the personal form which is attributed to them in the Timaeus, the logical character which they assume in the Sophist and Philebus, and the allusion to them in the Laws. In the Cratylus they dawn upon him with the freshness of a newly-discovered thought.

The Meno goes back to a former state of existence, in which men did and suffered good and evil, and received the reward or punishment of them until their sin was purged away and they were allowed to return to earth. This is a tradition of the olden time, to which priests and poets bear witness. The souls of men returning to earth bring back a latent memory of ideas, which were known to them in a former state. The recollection is awakened into life and consciousness by the sight of the things which resemble them on earth. The soul evidently possesses such innate ideas before she has had time to acquire them. This is proved by an experiment tried on one of Meno's slaves, from whom Socrates elicits truths of arithmetic and geometry, which he had never learned in this world. He must therefore have brought them with him from another.

The notion of a previous state of existence is found in the verses of Empedocles and in the fragments of Heracleitus. It was the natural answer to two questions, 'Whence came the soul? What is the origin of evil?' and prevailed far and wide in the east. It found its way into Hellas probably through the medium of Orphic and Pythagorean rites and mysteries. It was easier to think of a former than of a future life, because such a life has really existed for the race though not for the individual, and all men come into the world, if not 'trailing clouds of glory,' at any rate able to enter into the inheritance of the past. In the Phaedrus, as well as in the Meno, it is this former rather than a future life on which Plato is disposed to dwell. There the Gods, and men following in their train, go forth to contemplate the heavens, and are borne round in the revolutions of them. There they see the divine forms of justice, temperance, and the like, in their unchangeable beauty, but not without an effort more than human. The soul of man is likened to a charioteer and two steeds, one mortal, the other immortal. The charioteer and the mortal steed are in fierce conflict; at length the animal principle is finally overpowered, though not extinguished, by the combined energies of the passionate and rational elements. This is one of those passages in Plato which, partaking both of a philosophical and poetical character, is necessarily indistinct and inconsistent. The magnificent figure under which the nature of the soul is described has not much to do with the popular doctrine of the ideas. Yet there is one little trait in the description which shows that they are present to Plato's mind, namely, the remark that the soul, which had seen truths in the form of the universal, cannot again return to the nature of an animal.

In the Phaedo, as in the Meno, the origin of ideas is sought for in a previous state of existence. There was no time when they could have been acquired in this life, and therefore they must have been recovered from another. The process of recovery is no other than the ordinary law of association, by which in daily life the sight of one thing or person recalls another to our minds, and by which in scientific enquiry from any part of knowledge we may be led on to infer the whole. It is also argued that ideas, or rather ideals, must be derived from a previous state of existence because they are more perfect than the sensible forms of them which are given by experience. But in the Phaedo the doctrine of ideas is subordinate to the proof of the immortality of the soul. 'If the soul existed in a previous state, then it will exist in a future state, for a law of alternation pervades all things.' And, 'If the ideas exist, then the soul exists; if not, not.' It is to be observed, both in the Meno and the Phaedo, that Socrates expresses himself with diffidence. He speaks in the Phaedo of the words with which he has comforted himself and his friends, and will not be too confident that the description which he has given of the soul and her mansions is exactly true, but he 'ventures to think that something of the kind is true.' And in the Meno, after dwelling upon the immortality of the soul, he adds, 'Of some things which I have said I am not altogether confident' (compare Apology; Gorgias). From this class of uncertainties he exempts the difference between truth and appearance, of which he is absolutely convinced.

In the Republic the ideas are spoken of in two ways, which though not contradictory are different. In the tenth book they are represented as the genera or general ideas under which individuals having a common name are contained. For example, there is the bed which the carpenter makes, the picture of the bed which is drawn by the painter, the bed existing in nature of which God is the author. Of the latter all visible beds are only the shadows or reflections. This and similar illustrations or explanations are put forth, not for their own sake, or as an exposition of Plato's theory of ideas, but with a view of showing that poetry and the mimetic arts are concerned with an inferior part of the soul and a lower kind of knowledge. On the other hand, in the 6th and 7th books of the Republic we reach the highest and most perfect conception, which Plato is able to attain, of the nature of knowledge. The ideas are now finally seen to be one as well as many, causes as well as ideas, and to have a unity which is the idea of good and the cause of all the rest. They seem, however, to have lost their first aspect of universals under which individuals are contained, and to have been converted into forms of another kind, which are inconsistently regarded from the one side as images or ideals of justice, temperance, holiness and the like; from the other as hypotheses, or mathematical truths or principles.

In the Timaeus, which in the series of Plato's works immediately follows the Republic, though probably written some time afterwards, no mention occurs of the doctrine of ideas. Geometrical forms and arithmetical ratios furnish the laws according to which the world is created. But though the conception of the ideas as genera or species is forgotten or laid aside, the distinction of the visible and intellectual is as firmly maintained as ever. The IDEA of good likewise disappears and is superseded by the conception of a personal God, who works according to a final cause or principle of goodness which he himself is. No doubt is expressed by Plato, either in the Timaeus or in any other dialogue, of the truths which he conceives to be the first and highest. It is not the existence of God or the idea of good which he approaches in a tentative or hesitating manner, but the investigations of physiology. These he regards, not seriously, as a part of philosophy, but as an innocent recreation (Tim.).

Passing on to the Parmenides, we find in that dialogue not an exposition or defence of the doctrine of ideas, but an assault upon them, which is put into the mouth of the veteran Parmenides, and might be ascribed to Aristotle himself, or to one of his disciples. The doctrine which is assailed takes two or three forms, but fails in any of them to escape the dialectical difficulties which are urged against it. It is admitted that there are ideas of all things, but the manner in which individuals partake of them, whether of the whole or of the part, and in which they become like them, or how ideas can be either within or without the sphere of human knowledge, or how the human and divine can have any relation to each other, is held to be incapable of explanation. And yet, if there are no universal ideas, what becomes of philosophy? (Parmenides.) In the Sophist the theory of ideas is spoken of as a doctrine held not by Plato, but by another sect of philosophers, called 'the Friends of Ideas,' probably the Megarians, who were very distinct from him, if not opposed to him (Sophist). Nor in what may be termed Plato's abridgement of the history of philosophy (Soph.), is any mention made such as we find in the first book of Aristotle's Metaphysics, of the derivation of such a theory or of any part of it from the Pythagoreans, the Eleatics, the Heracleiteans, or even from Socrates. In the Philebus, probably one of the latest of the Platonic Dialogues, the conception of a personal or semi-personal deity expressed under the figure of mind, the king of all, who is also the cause, is retained. The one and many of the Phaedrus and Theaetetus is still working in the mind of Plato, and the correlation of ideas, not of 'all with all,' but of 'some with some,' is asserted and explained. But they are spoken of in a different manner, and are not supposed to be recovered from a former state of existence. The metaphysical conception of truth passes into a psychological one, which is continued in the Laws, and is the final form of the Platonic philosophy, so far as can be gathered from his own writings (see especially Laws). In the Laws he harps once more on the old string, and returns to general notions:—these he acknowledges to be many, and yet he insists that they are also one. The guardian must be made to recognize the truth, for which he has contended long ago in the Protagoras, that the virtues are four, but they are also in some sense one (Laws; compare Protagoras).

So various, and if regarded on the surface only, inconsistent, are the statements of Plato respecting the doctrine of ideas. If we attempted to harmonize or to combine them, we should make out of them, not a system, but the caricature of a system. They are the ever-varying expression of Plato's Idealism. The terms used in them are in their substance and general meaning the same, although they seem to be different. They pass from the subject to the object, from earth (diesseits) to heaven (jenseits) without regard to the gulf which later theology and philosophy have made between them. They are also intended to supplement or explain each other. They relate to a subject of which Plato himself would have said that 'he was not confident of the precise form of his own statements, but was strong in the belief that something of the kind was true.' It is the spirit, not the letter, in which they agree—the spirit which places the divine above the human, the spiritual above the material, the one above the many, the mind before the body.

The stream of ancient philosophy in the Alexandrian and Roman times widens into a lake or sea, and then disappears underground to reappear after many ages in a distant land. It begins to flow again under new conditions, at first confined between high and narrow banks, but finally spreading over the continent of Europe. It is and is not the same with ancient philosophy. There is a great deal in modern philosophy which is inspired by ancient. There is much in ancient philosophy which was 'born out of due time; and before men were capable of understanding it. To the fathers of modern philosophy, their own thoughts appeared to be new and original, but they carried with them an echo or shadow of the past, coming back by recollection from an elder world. Of this the enquirers of the seventeenth century, who to themselves appeared to be working out independently the enquiry into all truth, were unconscious. They stood in a new relation to theology and natural philosophy, and for a time maintained towards both an attitude of reserve and separation. Yet the similarities between modern and ancient thought are greater far than the differences. All philosophy, even that part of it which is said to be based upon experience, is really ideal; and ideas are not only derived from facts, but they are also prior to them and extend far beyond them, just as the mind is prior to the senses.

Early Greek speculation culminates in the ideas of Plato, or rather in the single idea of good. His followers, and perhaps he himself, having arrived at this elevation, instead of going forwards went backwards from philosophy to psychology, from ideas to numbers. But what we perceive to be the real meaning of them, an explanation of the nature and origin of knowledge, will always continue to be one of the first problems of philosophy.

Plato also left behind him a most potent instrument, the forms of logic—arms ready for use, but not yet taken out of their armoury. They were the late birth of the early Greek philosophy, and were the only part of it which has had an uninterrupted hold on the mind of Europe. Philosophies come and go; but the detection of fallacies, the framing of definitions, the invention of methods still continue to be the main elements of the reasoning process.

Modern philosophy, like ancient, begins with very simple conceptions. It is almost wholly a reflection on self. It might be described as a quickening into life of old words and notions latent in the semi-barbarous Latin, and putting a new meaning into them. Unlike ancient philosophy, it has been unaffected by impressions derived from outward nature: it arose within the limits of the mind itself. From the time of Descartes to Hume and Kant it has had little or nothing to do with facts of science. On the other hand, the ancient and mediaeval logic retained a continuous influence over it, and a form like that of mathematics was easily impressed upon it; the principle of ancient philosophy which is most apparent in it is scepticism; we must doubt nearly every traditional or received notion, that we may hold fast one or two. The being of God in a personal or impersonal form was a mental necessity to the first thinkers of modern times: from this alone all other ideas could be deduced. There had been an obscure presentiment of 'cognito, ergo sum' more than 2000 years previously. The Eleatic notion that being and thought were the same was revived in a new form by Descartes. But now it gave birth to consciousness and self-reflection: it awakened the 'ego' in human nature. The mind naked and abstract has no other certainty but the conviction of its own existence. 'I think, therefore I am;' and this thought is God thinking in me, who has also communicated to the reason of man his own attributes of thought and extension—these are truly imparted to him because God is true (compare Republic). It has been often remarked that Descartes, having begun by dismissing all presuppositions, introduces several: he passes almost at once from scepticism to dogmatism. It is more important for the illustration of Plato to observe that he, like Plato, insists that God is true and incapable of deception (Republic)—that he proceeds from general ideas, that many elements of mathematics may be found in him. A certain influence of mathematics both on the form and substance of their philosophy is discernible in both of them. After making the greatest opposition between thought and extension, Descartes, like Plato, supposes them to be reunited for a time, not in their own nature but by a special divine act (compare Phaedrus), and he also supposes all the parts of the human body to meet in the pineal gland, that alone affording a principle of unity in the material frame of man. It is characteristic of the first period of modern philosophy, that having begun (like the Presocratics) with a few general notions, Descartes first falls absolutely under their influence, and then quickly discards them. At the same time he is less able to observe facts, because they are too much magnified by the glasses through which they are seen. The common logic says 'the greater the extension, the less the comprehension,' and we may put the same thought in another way and say of abstract or general ideas, that the greater the abstraction of them, the less are they capable of being applied to particular and concrete natures.

Not very different from Descartes in his relation to ancient philosophy is his successor Spinoza, who lived in the following generation. The system of Spinoza is less personal and also less dualistic than that of Descartes. In this respect the difference between them is like that between Xenophanes and Parmenides. The teaching of Spinoza might be described generally as the Jewish religion reduced to an abstraction and taking the form of the Eleatic philosophy. Like Parmenides, he is overpowered and intoxicated with the idea of Being or God. The greatness of both philosophies consists in the immensity of a thought which excludes all other thoughts; their weakness is the necessary separation of this thought from actual existence and from practical life. In neither of them is there any clear opposition between the inward and outward world. The substance of Spinoza has two attributes, which alone are cognizable by man, thought and extension; these are in extreme opposition to one another, and also in inseparable identity. They may be regarded as the two aspects or expressions under which God or substance is unfolded to man. Here a step is made beyond the limits of the Eleatic philosophy. The famous theorem of Spinoza, 'Omnis determinatio est negatio,' is already contained in the 'negation is relation' of Plato's Sophist. The grand description of the philosopher in Republic VI, as the spectator of all time and all existence, may be paralleled with another famous expression of Spinoza, 'Contemplatio rerum sub specie eternitatis.' According to Spinoza finite objects are unreal, for they are conditioned by what is alien to them, and by one another. Human beings are included in the number of them. Hence there is no reality in human action and no place for right and wrong. Individuality is accident. The boasted freedom of the will is only a consciousness of necessity. Truth, he says, is the direction of the reason towards the infinite, in which all things repose; and herein lies the secret of man's well-being. In the exaltation of the reason or intellect, in the denial of the voluntariness of evil (Timaeus; Laws) Spinoza approaches nearer to Plato than in his conception of an infinite substance. As Socrates said that virtue is knowledge, so Spinoza would have maintained that knowledge alone is good, and what contributes to knowledge useful. Both are equally far from any real experience or observation of nature. And the same difficulty is found in both when we seek to apply their ideas to life and practice. There is a gulf fixed between the infinite substance and finite objects or individuals of Spinoza, just as there is between the ideas of Plato and the world of sense.

Removed from Spinoza by less than a generation is the philosopher Leibnitz, who after deepening and intensifying the opposition between mind and matter, reunites them by his preconcerted harmony (compare again Phaedrus). To him all the particles of matter are living beings which reflect on one another, and in the least of them the whole is contained. Here we catch a reminiscence both of the omoiomere, or similar particles of Anaxagoras, and of the world-animal of the Timaeus.

In Bacon and Locke we have another development in which the mind of man is supposed to receive knowledge by a new method and to work by observation and experience. But we may remark that it is the idea of experience, rather than experience itself, with which the mind is filled. It is a symbol of knowledge rather than the reality which is vouchsafed to us. The Organon of Bacon is not much nearer to actual facts than the Organon of Aristotle or the Platonic idea of good. Many of the old rags and ribbons which defaced the garment of philosophy have been stripped off, but some of them still adhere. A crude conception of the ideas of Plato survives in the 'forms' of Bacon. And on the other hand, there are many passages of Plato in which the importance of the investigation of facts is as much insisted upon as by Bacon. Both are almost equally superior to the illusions of language, and are constantly crying out against them, as against other idols.

Locke cannot be truly regarded as the author of sensationalism any more than of idealism. His system is based upon experience, but with him experience includes reflection as well as sense. His analysis and construction of ideas has no foundation in fact; it is only the dialectic of the mind 'talking to herself.' The philosophy of Berkeley is but the transposition of two words. For objects of sense he would substitute sensations. He imagines himself to have changed the relation of the human mind towards God and nature; they remain the same as before, though he has drawn the imaginary line by which they are divided at a different point. He has annihilated the outward world, but it instantly reappears governed by the same laws and described under the same names.

A like remark applies to David Hume, of whose philosophy the central principle is the denial of the relation of cause and effect. He would deprive men of a familiar term which they can ill afford to lose; but he seems not to have observed that this alteration is merely verbal and does not in any degree affect the nature of things. Still less did he remark that he was arguing from the necessary imperfection of language against the most certain facts. And here, again, we may find a parallel with the ancients. He goes beyond facts in his scepticism, as they did in their idealism. Like the ancient Sophists, he relegates the more important principles of ethics to custom and probability. But crude and unmeaning as this philosophy is, it exercised a great influence on his successors, not unlike that which Locke exercised upon Berkeley and Berkeley upon Hume himself. All three were both sceptical and ideal in almost equal degrees. Neither they nor their predecessors had any true conception of language or of the history of philosophy. Hume's paradox has been forgotten by the world, and did not any more than the scepticism of the ancients require to be seriously refuted. Like some other philosophical paradoxes, it would have been better left to die out. It certainly could not be refuted by a philosophy such as Kant's, in which, no less than in the previously mentioned systems, the history of the human mind and the nature of language are almost wholly ignored, and the certainty of objective knowledge is transferred to the subject; while absolute truth is reduced to a figment, more abstract and narrow than Plato's ideas, of 'thing in itself,' to which, if we reason strictly, no predicate can be applied.

The question which Plato has raised respecting the origin and nature of ideas belongs to the infancy of philosophy; in modern times it would no longer be asked. Their origin is only their history, so far as we know it; there can be no other. We may trace them in language, in philosophy, in mythology, in poetry, but we cannot argue a priori about them. We may attempt to shake them off, but they are always returning, and in every sphere of science and human action are tending to go beyond facts. They are thought to be innate, because they have been familiar to us all our lives, and we can no longer dismiss them from our mind. Many of them express relations of terms to which nothing exactly or nothing at all in rerum natura corresponds. We are not such free agents in the use of them as we sometimes imagine. Fixed ideas have taken the most complete possession of some thinkers who have been most determined to renounce them, and have been vehemently affirmed when they could be least explained and were incapable of proof. The world has often been led away by a word to which no distinct meaning could be attached. Abstractions such as 'authority,' 'equality,' 'utility,' 'liberty,' 'pleasure,' 'experience,' 'consciousness,' 'chance,' 'substance,' 'matter,' 'atom,' and a heap of other metaphysical and theological terms, are the source of quite as much error and illusion and have as little relation to actual facts as the ideas of Plato. Few students of theology or philosophy have sufficiently reflected how quickly the bloom of a philosophy passes away; or how hard it is for one age to understand the writings of another; or how nice a judgment is required of those who are seeking to express the philosophy of one age in the terms of another. The 'eternal truths' of which metaphysicians speak have hardly ever lasted more than a generation. In our own day schools or systems of philosophy which have once been famous have died before the founders of them. We are still, as in Plato's age, groping about for a new method more comprehensive than any of those which now prevail; and also more permanent. And we seem to see at a distance the promise of such a method, which can hardly be any other than the method of idealized experience, having roots which strike far down into the history of philosophy. It is a method which does not divorce the present from the past, or the part from the whole, or the abstract from the concrete, or theory from fact, or the divine from the human, or one science from another, but labours to connect them. Along such a road we have proceeded a few steps, sufficient, perhaps, to make us reflect on the want of method which prevails in our own day. In another age, all the branches of knowledge, whether relating to God or man or nature, will become the knowledge of 'the revelation of a single science' (Symp.), and all things, like the stars in heaven, will shed their light upon one another.