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.