§15. _A rebus ... involutis_: physical phenomena are often spoken of in these words by Cic., cf. 19, _Timaeus_ c. 1, _D.F._ I. 64, IV. 18, V. 10, _N.D._ I. 49. Ursinus rejected _ab_ here, but the insertion or omission of _ab_ after the passive verb depends on the degree to which _natura_ is personified, if 28 be compared with _Tim._ c. 1, this will be clear. _Involutis_ = veiled; cf. _involucrum_. Cic. shows his feeling of the metaphor by adding _quasi_ in II. 26, and often. _Avocavisse philosophiam_: this, the Xenophontic view of Socrates, was the popular one in Cicero's time, cf. II. 123, _T.D._ V. 10, _D.F._ V. 87, 88, also Varro in Aug. _De Civ. Dei_, VIII. 3. Objections to it, however occurred to Cic., and were curiously answered in _De Rep._ I. 16 (cf. also Varro in Aug. _De Civ. Dei_, VIII. 4). The same view is supposed to be found in Aristotle, see the passages quoted by R. and P. 141. To form an opinion on this difficult question the student should read Schleiermacher's _Essay on the Worth of Socrates as a Philosopher_ (trans. by Thirlwall), and Zeller's _Socrates and the Socratic Schools_, Eng. Trans., pp. 112--116 [I dissent from his view of Aristotle's evidence], also Schwegler's _Handbook_, so far as it relates to Socrates and Plato. _Nihil tamen ad bene vivendum valere_: _valere_ is absent from MSS., and is inserted by Halm, its use in 21 makes it more probable than _conferre_, which is in ed. Rom. (1471). Gronovius vainly tries to justify the MSS. reading by such passages as _D.F._ I. 39, _T.D._ I. 70. The strangest ellipse with _nihil ad_ elsewhere in Cic. is in _De Leg._ I. 6.