But the efforts of ancient and modern philosophers rigorously to define empirical concepts afford more interest than the external form of treatment, as do their efforts to modify or to simplify, or indeed finally to deduce them rationally. The Platonic dialogues, such as the _Charmides,_ the _Lachetes,_ the _Protagoras,_ are most instructive in this respect. Here it is sought to define sophrosune, andreia and the other virtues, without arriving at any precise result, or rather arriving at the contradictory one, that each of these virtues is _the whole of virtue,_ whereas it should only be a _part_ of it. In the _Republic_ is sought the relation of the four virtues, or rather of three of them, prudence, temperance, and fortitude, with justice, which forms as it were the foundation and unity of the whole. From such discussions arose the affirmation, to be found also in Cicero, that the virtues are inseparable from one another: _virtutes ita copulatae connexaeque sunt, ut omnes participes sint, nec alia ab alia possit separari._[10] The difficulty of the Platonic inquiry is renewed with all those who have given definitions of the virtues and of the other empirical concepts, because, when they have achieved with much labour a definition which appears satisfactory, it is afterwards always found to be too narrow or too wide. Thus the definition given by Kant and by others (Fichte, Schopenhauer) of egoism, consisting in their view, of considering other individuals as means and not ends, is the definition, not of egoism, but of any form of immorality which debases the Spirit that should be the end, by means of its own caprices. The same is to be said of the definitions given by Fichte as to the duties inherent to this or that condition and state: the duties, for instance, of the learned, who should love truth, communicate it to others, rectify errors, promote culture,[11] and so on. These are all things that form part of the duty, not only of the learned, but of every man. The simplifiers are not more fortunate in their attempts to reduce the number of empirical concepts, for the concepts excluded by them have neither more nor less right to recognition than the others that they have accepted. Schopenhauer, for instance, when he rejects the class of duties toward oneself,[12] should also reject that of duties toward others. For others and ourselves are correlative terms, and we cannot be benevolent to others and malevolent to ourselves, just to others and unjust to ourselves. If this be met with the objection that the empirical self is not the object of duties, we must reply that neither are the empirical "others," but only that Spirit which is in all and constitutes all. In reacting against these unifiers and simplifiers, other philosophers (as for example Herbart) have maintained the indeducibility of the virtues or duties from a single principle, which means that they have received those concepts into their philosophy