HIS life of our bodies, material and subject to flux, always advancing by way of motion, finds the power of its being in this, that it never rests from its motion: and as some river, flowing on by its own impulse, keeps the channel in which it runs well filled, yet is not seen in the same water always at the same place, but part of it glides away while part comes flowing on, so, too, the material element of our life here suffers change in the continuity of its succession of opposites by way of motion and flux, so that it never can desist from change, but in its inability to rest keeps up unceasingly its motion alternating by like ways : and if it should ever cease moving it will assuredly have cessation also of its being.

For instance, emptying succeeds fulness, and on the other hand after emptiness comes in turn a process of filling: sleep relaxes the strain of waking, and, again, awakening braces up what had become slack: and neither of these abides continually, but both give way, each at the other's coming; nature thus by their interchange so renewing herself as, while partaking of each in turn, to pass from the one to the other without break. For that the living creature should always be exerting itself in its operations produces a certain rupture and severance of the overstrained part; and continual quiescence of the body brings about a certain dissolution and laxity in its frame: but to be in touch with each of these at the proper times in a moderate degree is a staying-power of nature, which, by continual transference to the opposed states, gives herself in each of them rest from the other. Thus she finds the body on the strain through wakefulness, and devises relaxation for the strain by means of sleep, giving the perceptive faculties rest for the time from their operations, loosing them like horses from the chariots after the race.

Περὶ ὕπνου καὶ χάσμης καὶ ὀνείρων αἰτιολογία

Ἡ ὑλικὴ καὶ ῥοώδης αὕτη τῶν σωμάτων ζωή͵ πάντοτε διὰ κινήσεως προϊοῦσα͵ ἐν τούτῳ ἔχει τοῦ εἶναι τὴν δύναμιν͵ ἐν τῷ μὴ στῆναί ποτε τῆς κινήσεως. Καθάπερ δέ τις ποταμὸς κατὰ τὴν ἰδίαν ῥέων ὁρμὴν͵ πλήρη μὲν δείκνυσι τὴν κοιλότητα δι' ἧς ἂν τύχῃ φερόμενος͵ οὐ μὴν ἐν τῷ αὐτῷ ὕδατι περὶ τὸν αὐτὸν ἀεὶ τόπον ὁρᾶται͵ ἀλλὰ τὸ μὲν ὑπέδραμεν αὐτοῦ͵ τὸ δὲ ὑπεῤῥέει, οὕτω καὶ τὸ ὑλικὸν τῆς τῇδε ζωῆς διά τινος κινήσεως καὶ ῥοῆς τῇ συνεχείᾳ τῆς τῶν ἐναντίων διαδοχῆς ἀμείβεται͵ ὡς ἂν μηδέποτε στῆναι δύνασθαι τῆς μεταβολῆς͵ ἀλλὰ τῇ δυνάμει τοῦ ἀτρεμεῖν ἄπαυστον ἔχειν διὰ τῶν ὁμοίων ἐναμειβομένην τὴν κίνησιν. Εἰ δέ ποτε κινούμενον παύσοιτο͵ καὶ τοῦ εἶναι πάντως τὴν παῦλαν ἕξει·

οἷον͵ διεδέξατο τὸ πλῆρες ἡ κένωσις καὶ πάλιν ἀντεισῆλθεν ἡ πλήρωσις τῇ κενότητι. Ὕπνος τὸ σύντονον τῆς ἐγρηγόρσεως ὑπεχάλασεν͵ εἶτα ἐγρήγορσις τὸ ἀνειμένον ἐτόνωσε. Καὶ οὐδέτερον τούτων ἐν τῷ διηνεκεῖ συμμένει͵ ἀλλ' ὑποχωρεῖ ταῖς παρουσίαις ἀλλήλων ἀμφότερα͵ οὕτω τῆς φύσεως ἑαυτὴν ταῖς ὑπαλλαγαῖς ἀνακαινιζούσης͵ ὡς ἑκατέρων ἐν τῷ μέρει μεταλαγχάνουσαν ἀδιασπάστως ἀπὸ τοῦ ἑτέρου μεταβαίνειν ἐπὶ τὸ ἕτερον. Τό τε γὰρ διαπαντὸς συντετάσθαι ταῖς ἐνεργείαις τὸ ζῶον͵ ῥῆξίν τινα καὶ διασπασμὸν τῶν ὑπερτεινομένων ποιεῖται μερῶν, ἥ τε διηνεκὴς τοῦ σώματος ἄνεσις διάπτωσίν τινα τοῦ συνεστῶτος καὶ λύσιν ἐργάζεται. Τὸ δὲ κατὰ καιρὸν μετρίως ἑκατέρων ἐπιτυγχάνειν͵ δύναμις πρὸς διαμονήν ἐστι τῆς φύσεως͵ διὰ τῆς διηνεκοῦς πρὸς τὰ ἀντικείμενα μεταβάσεως ἐν ἑκατέροις ἑαυτὴν ἀπὸ τῶν ἑτέρων ἀναπαυούσης. Οὕτω τοίνυν τετονωμένον διὰ τῆς ἐγρηγόρσεως τὸ σῶμα λαβοῦσα͵ λύσιν ἐμποιεῖ διὰ τοῦ ὕπνου τῷ τόνῳ͵ τὰς αἰσθητικὰς δυνάμεις πρὸς καιρὸν ἐκ τῶν ἐνεργειῶν ἀναπαύσασα͵ οἷόν τινας ἵππους μετὰ τοὺς ἀγῶνας τῶν ἁρμάτων ἐκλύσασα.