Moreover, no one of the other categories is separable [or independent]. This is attested by the procedure of the older philosophers; for it was the principles, elements, and causes of essence that were the objects of their investigations. The thinkers of the present day, to be sure, are rather inclined to consider universals as essence. For genera are universals, and these they hold to be principles and essences, mainly because their mode of investigation is a logical one. The older philosophers, on the other hand, considered particular things to be essences; e.g., fire and earth, not body in general.
There are three essences. Two of these are sensible, one being eternal and the other transient. The latter is obvious to all, in the form of plants and animals; with regard to the former, there is room for discussion, as to whether its elements are one or many. The third, differing from the other two, is immutable and is maintained by certain persons to be separable. Some make two divisions of it, whereas others class together, as of one nature, ideas and mathematical entities; and others again admit only the latter. The first two essences belong to physical science, for they are subject to change; the last belongs to another science, if there is no principle common to all.
ON COMMUNITY OF STUDIES
From 'The Politics,' Book 8
No one, therefore, can doubt that the legislator ought principally to attend to the education of youth. For in cities where this is neglected, the politics are injured. For every State ought to be governed according to its nature; since the appropriate manners of each polity usually preserve the polity, and establish it from the beginning. Thus, appropriate democratic manners preserve and establish a democracy, and oligarchic an oligarchy. Always, however, the best manners are the cause of the best polity. Further still, in all professions and arts, there are some things which ought previously to be learnt, and to which it is requisite to be previously accustomed, in order to the performance of their several works,; so that it is evident that it is also necessary in the practice of virtue.
Since, however, there is one purpose to every city, it is evident that the education must necessarily be one and the same in all cities; and that the attention paid to this should be common. At the same time, also, no one ought to think that any person takes care of the education of his children separately, and privately teaches them that particular discipline which appears to him to be proper. But it is necessary that the studies of the public should be common. At the same time, also, no one ought to think that any citizen belongs to him in particular, but that all the citizens belong to the city; for each individual is a part of the city. The care and attention, however, which are paid to each of the parts, naturally look to the care and attention of the whole. And for this, some one may praise the Lacedaemonians; for they pay very great attention to their children, and this in common. It is evident, therefore, that laws should be established concerning education, and that it should be made common.
Virtue, to men thou bringest care and toil;
Yet art thou life's best, fairest spoil!