deprive me of these ideas, prevent me from noting any resemblance or any difference between the red and the blue, and although my sensations of these colours would remain, my knowledge of them would depart—so essential are ideas to the existence of, so impotent are sensations, without ideas, to instruct us even in the most elementary truths. This case may further serve to illustrate a subject on which Plato has bestowed a good deal of elaborate treatment—the conversion, namely, of the human soul from ignorance to true knowledge. The ignorant and unconverted soul supposes that its knowledge of colours is due to the impressions which it receives. The converted soul is aware that this knowledge is due, not to these impressions, but to the ideas of resemblance and difference (and some other ideas), by which these impressions are accompanied, but with which they are not by any means identical. Apply this doctrine to the whole sensible phenomena of the universe, and it will be seen that we learn nothing from them, but that our knowledge of outward things is based entirely upon ideas, and is effected solely by their mediation.
Perhaps some light may be thrown on the Platonic and the sensational theories, if we bring out the opposition between them by considering the difference between mere]y feeling a sensation and thinking a sensation. The person who merely feels a sensation, and from whose mind every trace of thought is supposed to be banished, cannot travel mentally one hairsbreadth beyond the sensation which engrosses him. However keen the sensation may be, he is tied down rigorously to that single experience, and can take nothing else into account; for sensation cannot take into account anything except itself. And suppose that this person experiences another or twenty other sensations, still so long as he is without ideas he is just where he was when he had only one sensation. He is in a state of blank unintelligence, for he cannot, by means of mere sensation, so pass from one sensation to another as to make any comparison amongst them. Lively as his sensations may be, no knowledge of them has as yet taken place, no knowledge as yet is possible. But now let this person think his sensation, instead of merely feeling it, and observe what happens. His mind goes beyond the sensation, and takes in something more. He refers the sensation to a class; he brings into connection with it something different from itself. This he does in simply thinking that it is; for Being is no sensation; it is a thought. To explicate completely the difference between thought and sensation would carry us much too far. But this fact is certain, that in thinking a sensation something is present to the mind, which is not present to it when it merely feels the sensation. That something is a Platonic idea. And thus the doctrine which would build up knowledge out of mere sensations is displaced. The signal importance of ideas is the less readily appreciated from its being impossible for us to realize actually the sensational condition as it is when deprived altogether of their light. This can only be surmised or reached by the way of supposition.
Ideas may be further explained by observing that what is present to the mind when, instead of merely feeling, it thinks and knows, is a class, genus, or species. All general conceptions, such as man, animal, tree, are ideas; they are also called universals, to distinguish them from the particulars which are included under them. The modern logical theory of general conceptions may be here contrasted with the Platonic doctrine. According to the modern doctrine, the mind commences with a knowledge of particulars and then proceeds, by a method of abstraction and generalization (which consists in attending to agreements and leaving out of view differences), to fabricate general conceptions or. Here two errors are committed. First, it is impossible for knowledge to commence with particulars, for particulars can only be known or thought of in the act which assigns them to a class; and, secondly, the problem being. What is the origin of our knowledge? this explanation, leaving that problem unresolved, merely explains our ideas as arising out of our knowledge! The Platonic doctrine is very different, and much more to the purpose. According to Plato, the first stage of knowledge is, not the apprehension of particulars, but the apprehension of ideas or universals, and the application of these to particulars. This solution, at any rate, meets the problem, because it makes knowledge to originate in ideas, and not ideas to have their origin in knowledge. The Platonic theory may be summed up by saying, that the mind thinks and knows by means of genera and species. These are the laws under which all intelligence must work. They are the essential conditions of all thought, all knowledge, and all existence. It is impossible for a thing either to exist, or to be known, except as an instance of some genus or species. Genera and species—in other words, ideas—are thus the most objective, the most independent, the most real, and the most enduring of all things, inasmuch as they are the necessary laws or principles on which all being and all knowing are dependent. Such is the realism of Plato—a doctrine much truer and more profound than either the nominalism or conceptualism by which it has been succeeded.
The physics of Plato may be passed over as presenting few points of interest or intelligibility. His ethics have a stronger claim on our attention. Plato's moral philosophy will be best understood by being confronted with that of the sophists, against which it was specially directed, just as his theory of ideas was designed to refute their theory of knowledge. If man be nothing but an aggregate of sensations, he can have no other end than sensational enjoyment, and no other principle of action than selfishness. Such, accordingly, was the general purport of the sophistical morality, although some of its expounders recoiled from the extreme conclusions to which their principles led. Others, however, were less scrupulous. They explained the origin of justice in this curious fashion. The best condition, they said, in which a man can be placed, is that in which he can injure others with impunity; the worst is that in which he can be injured without the power of defence or retaliation. But men cannot always assure themselves of the best condition, or guard against falling into the worst. This consideration leads them to a com promise, in which they consent to abandon the former condition in order to escape the latter, the evils of which outweigh the advantages of the other state. This compromise is itself justice, and such are the circumstances in which that virtue originates. From this it follows that the semblance of justice is better than the reality; because the semblance will prevent others from injuring us, while it will yet enable us to injure them to our heart's content—(Republic, p. 358, 9.) In answer to this sophistical deduction, Plato argues that justice is not (as this doctrine assumes) an unessential attribute, but is itself the essence and organization of the soul. The semblance of justice, he says, without the reality, is no more a good thing for its possessor, than the semblance of order is a good thing in a nation when all its ranks are in a condition of anarchy and rebellion, or than the appearance of health is a good thing in the human body when all its organs are really in a state of disease. It is principally for the purpose of showing that virtue must be a reality, and not a sham, that Plato, in his "Republic," has drawn a parallel between the soul of man and the political constitution of a state. Just as a state cannot exist unless it is sustained by political justice, that is to say, unless the rightful rulers rule, and are aided by the military, and unless the inferior orders obey; so the individual soul does not truly and healthfully exist unless it is the embodiment of private or personal justice, that is to say, unless reason rules the lower appetites, and is aided in its government by the more heroic passions of our nature. In short, just as a state without justice—that is, without the due subjection of the governed to the governing powers—is a state disorganized; so a soul without justice—that is, without the proper subordination of the inferior to the superior principles of our constitution— is a soul undone. A character which wears the mask without having the substance of virtue, is no better, indeed is worse off, than a sick body which presents the mere appearance of health. Such is the scope (in so far as a few sentences can give it) of the moral philosophy of Plato in its more popular aspect, as presented to us in the "Republic." He treats the subject more metaphysically in the "Philebus;" but the result reached is in both cases the same. The maintenance of that organization of the soul in which reason rules and passion obeys—this is the end to be aimed at by man, rather than happiness or pleasure.
But more important than any results, cither moral or metaphysical, which have been brought to maturity by Plato, are the inexhaustible germs of latent wealth which his writings contain. Every time his pages are turned, they throw forth new seeds of wisdom, new scintillations of thought—so teeming is the fertility, so irrepressible the fulness, of his genius. All philosophy, speculative and practical, has been foreshadowed by his prophetic intelligence, often dimly, but always so attractively as to whet the curiosity of those who have chosen him for their guide.