5. in the Logic of Aristotle,ὅπερ ἐστί,orὅπερalone, has two senses :
a. non-technical, and unemphatic,what(a thing)is, ἑκάστη οὐσία τοῦθ' ὅ. ἐστίν, οὐ λέγεται μᾶλλον καὶ ἧττονeach substance is calledwhatitiswithout the difference of more or less, Cat. 3b36 ;τὸ διπλάσιον τοῦθ' ὅ. ἐστίν, ἑτέρου λέγεταιthe double is calledwhatitis(viz. the double) of something, i.e. is relative, ib. 6a39 .
b. expressing identity,οὔτε ἡ χιὼν ὅ. λευκόνsnow is notwhatwhiteis,i.e. is not identical with white, Top. 120b23 ;ὁ λευκὸς ἄνθρωπος οὐκ ἔστιν ὅ. χρῶμαib. 116a27 : hence, to indicate thepreciseoressentialnature of a thing,οὐ γὰρ ἂν φαίη ὅ. κακόν τι εἶναι τὴν ἡδονήνhe would not say that pleasure isessentiallysomething bad, EN 1153b6 ;τὰ μὲν οὐσίαν σημαίνοντα ὅ. ἐκεῖνό ἢ ὅ. ἐκεῖνό τι σημαίνειexpressions which show the essence showprecisely whatthe thing in questionisorpreciselyof what kind itis(i.e. indicate either its species or its genus), APo. 83a24 ;ὅ. <τόδε> τί ἐστι τὸ τί ἦν εἶναιthe essence of a thingis preciselya 'this', i.e. a fully specified particular, Metaph. 1030a3 ;ἡ μὲν [ἐπιστήμη] ὅ. ἀνθρώπου ἐστίνknowledge (that man is an animal) is apprehension that 'animal' isan element in the essential natureof man, APo. 89a35 .