Epictetus, who alone amongst these later Stoics was an ardent student of dialectics, held fast to the main principle that certainty is attainable. ‘How indeed’ he said ‘perception is effected, whether through the whole body or any part, perhaps I cannot explain, for both opinions perplex me. But that you and I are not the same, I know with perfect certainty[100].’ [Sidenote: Grammar.] =160.= Having now dealt with the theory of knowledge, we may consider briefly the subordinate sciences (or rather ‘arts’) of Grammar, Logic (in the narrower sense), and Style. Here we may leave the technical divisions and subdivisions of the Stoics; for these matters are substantially independent of the main lines upon which the ancient philosophies parted company, and have for us only a secondary and historical interest. The Stoics distinguish five parts of speech: ‘name’ (ὄνομα, _nomen_), as ‘Diogenes’; ‘class-name’ (προσηγορία, _appellatio_), as ‘man, horse’[101]; ‘verb’ (ῥῆμα, _verbum_); ‘conjunction’ (σύνδεσμος, _coniunctio_); and ‘article’ (ἄρθρον, _articulus_). The last they define naïvely as a little word which is all ending, and serves to distinguish the cases and numbers[102]. To the list of the parts of speech Antipater added the ‘mixed part’ or participle (μεσότης). The noun has four cases (πτώσεις), the ‘upright case’ (πτῶσις εὐθεῖα, _casus rectus_; this is of course a contradiction in terms); and the ‘oblique’ cases (πλάγιαι), that is the ‘class’ case (γενική), the ‘dative’ (δοτική), and the ‘effect’ case (αἰτιατική). The ῥῆμα or verb is identical with the κατηγόρημα or ‘predicate,’ and may take the ‘active’ form (ὀρθά), the ‘passive’ (ὕπτια), or the ‘neuter’ (οὐδέτερα); some verbs also express action and reaction, and are called ‘reflexive’ (ἀντιπεπονθότα). The Stoics also distinguished the tenses. Time (χρόνος) being of three kinds, past (παρῳχημένος), present (ἐνεστώς), and future (μέλλων), we have the following tenses which are ‘definite’ (ὡρισμένοι): the ‘present imperfect’ (ἐνεστὼς ἀτελής), the ‘past imperfect’ (παρῳχημένος ἀτελής), the ‘present perfect’ (ἐνεστὼς τέλειος), and the ‘past perfect’ (παρῳχημένος τέλειος); in addition to these we have the ‘indefinite’ tenses, the future (μέλλων), and the past indefinite, called simply indefinite (ἀόριστος)[103]. [Sidenote: Theories of speech.]