_Science_ is, then, according to Plato, _the knowledge of universal, necessary, unchangeable, and eternal ideas_. The simple cognition of the concrete phenomena of the universe is not regarded by him as _real_ knowledge. "Science, or real knowledge, belongs to _Being_, and ignorance to _non_-Being." Whilst that which is conversant only "with that which partakes of both--of being and non-being--and which can not be said either to be or not to be"--that which is perpetually "becoming," but never "really is," is "simply _opinion_, and not real knowledge."[518] And those only are "philosophers" who have a knowledge of the _really-existing_, in opposition to the mere seeming; of the _always-existing_, in opposition to the transitory; and of that which exists _permanently_, in opposition to that which waxes and wanes--is developed and destroyed alternately. "Those who recognize many beautiful things, but who can not see the Beautiful itself, and can not even follow those who would lead them to it, they _opine_, but do not _know_. And the same may be said of those who recognize right actions, but do not recognize an absolute righteousness. And so of other ideas. But they who look at these ideas--permanent and unchangeable ideas--these men _really know_."[519] Those are the true philosophers alone who love the sight of truth, and who have attained to the vision of the eternal order, and righteousness, and beauty, and goodness in the Eternal Being. And the means by which the soul is raised to this vision of real Being (τὸ ὄντως ὄν) is THE SCIENCE OF REAL KNOWLEDGE. Plato, in the "Theætetus," puts this question by the interlocutor Socrates, "What is Science (᾽Επιστήμη) or positive knowledge?"[520] Theætetus essays a variety of answers, such as, "Science is sensation," "Science is right judgment or opinion," "Science is right opinion with logical definition." These, in the estimation of the Platonic Socrates, are all unsatisfactory and inadequate. But after you have toiled to the end of this remarkable discussion, in which Socrates demolishes all the then received theories of knowledge, he gives you no answer of his own. He abruptly closes the discussion by naïvely remarking that, at any rate, Theætetus will learn that he does not understand the subject; and the ground is now cleared for an original investigation. [Footnote 518: "Republic," bk. v. ch. xx.] [Footnote 519: Ibid., bk. v. ch. xxii.] [Footnote 520: "Theætetus," § 10.]