The proper way of answering this question has been much debated, from Plato’s day down to the present. It is one of the fundamental problems of Ethical Philosophy.
The subjective matter of fact, implied by every one who designates an act or a person as virtuous, is an approving or admiring sentiment which each man knows in his own bosom. But Plato assumes that there is, besides this, an objective connotation: a common object or property to which such sentiment refers. What is that common object? I see no other except that which is indicated by the principle of Utility: I mean that principle which points out Happiness and Unhappiness, not merely of the agent himself, but also of others affected or liable to be affected by his behaviour, as the standard to which these denominations refer. Courage, Prudence, Temperance, Justice, all tend to prevention and mitigation of unhappiness, and to increase of happiness, as well for the agent himself as for the society surrounding him. The opposite qualities — Timidity, Imprudence, Intemperance, Injustice — tend with equal certainty either to increase positively the unhappiness of the agent and of society, or to remove the means for warding it off or abating it. Indeed there is a certain minimum of all the four — Courage, Prudence, Temperance, Justice — without which or below which neither society could hold together, nor the life of the individual agent himself could be continued.
Tendency of the four opposite qualities to lessen human happiness.
Here then is one answer at least to the question of Plato. Courage, Prudence, Temperance, Justice — all of them mental attributes of rational voluntary agents — have also the common property of being, in a certain minimum degree, absolutely essential to the life of the agent and the maintenance of society — and of being, above that degree, tutelary against the suffering, and beneficial to the happiness, of both. This tutelary or beneficent tendency is the common objective property signified by the general term Virtue; and is implicated with the subjective property before mentioned — the sentiment of approbation. The four opposite qualities are designated by the general term Vice or Defect, connoting both maleficent tendency and the sentiment of disapprobation.
A certain measure of all the four virtues is required. In judging of particular acts instigated by each, there is always a tacit reference to the hurt or benefit in the special case.
This proposition will be farther confirmed, if we look at all the four qualities — Courage, Prudence, Temperance, Justice — in another point of view. Taking them in their reference to Virtue, each of them belongs to Virtue as a part to the whole,[495] not as one species contradistinguished from and excluding other species. The same person may have, and ought to have, a certain measure of all: he will not be called virtuous unless he has a measure of all. Excellence in any one will not compensate for the entire absence of the others.
[495] Compare Plato, Legg. i. p. 629 B, where he describes τὴν ξύμπασαν ἀρετὴν — δικαιοσύνη καὶ σωφροσύνη καὶ φρόνησις εἰς ταὐτὸν ἐλθοῦσα μετ’ ἀνδρείας: also pp. 630 C-E, 631 A, where he considers all these as μόρια ἀρετῆς, but φρόνησις as the first of the four and ἀνδρεία as the last.
See also iii. pp. 688 B, 696 C-D, iv. p. 705 D.
A just and temperate man will not be accounted virtuous, if (to use an Aristotelian simile) he be so extravagantly timid as to fear every insect that flits by, or the noise of a mouse.[496] All probability of beneficent results from his agency is effaced by this capital defect: and it is the probability of such results which constitute his title to be called virtuous.
[496] Aristot. Ethic. Nikomach. vii. 6, p. 1148, a. 8; Politic. vii. 1, p. 1323, a. 29. κἂν ψοφήσῃ μῦς … δεδιὼς τὰς παραπετομένας μυίας.