Simplikius (ad Aristot. Phys. fol. 6 A) describes Diogenes as having been σχεδὸν νεώτατος in the series of physical theorists.
Diogenes the Apolloniate, the latest in the series of Ionic philosophers or physiologists, adopted, with modifications and enlargements, the fundamental tenet of Anaximenes. There was but one primordial element — and that element was air. He laid it down as indisputable that all the different objects in this Kosmos must be at the bottom one and the same thing: unless this were the fact, they would not act upon each other, nor mix together, nor do good and harm to each other, as we see that they do. Plants would not grow out of the earth, nor would animals live and grow by nutrition, unless there existed as a basis this universal sameness of nature. No one thing therefore has a peculiar nature of its own: there is in all the same nature, but very changeable and diversified.[183]
[183] Diogen. Ap. Fragm. ii. c. 29 Panzerb.; Theophrastus, De Sensu, s. 39.
εἰ γὰρ τὰ ἐν τῷδε τῷ κόσμῳ ἐόντα νῦν γῆ καὶ ὕδωρ καὶ τἄλλα, ὅσα φαινεται ἐν τῷδε τῷ κόσμῳ ἐόντα, εἰ τουτέων τι ἦν τὸ ἕτερον τοῦ ἑτέρου ἕτερον ἐὸν τῇ ἰδίῃ φύσει, καὶ μὴ τὸ αὐτὸ ἐὸν μετέπιπτε πολλαχῶς καὶ ἡτεροιοῦτο· οὐδαμῆ οὔτε μίσγεσθαι ἀλλήλοις ἠδύνατο οὔτε ὠφέλησις τῷ ἑτέρῳ οὔτε βλάβη, &c.
Aristotle approves this fundamental tenet of Diogenes, the conclusion that there must be one common Something out of which all things came — ἐξ ἑνὸς ἅπαντα (Gen. et Corrupt. i. 6-7, p. 322, a. 14), inferred from the fact that they acted upon each other.
Air was the primordial, universal element.
Now the fundamental substance, common to all, was air. Air was infinite, eternal, powerful; it was, besides, full of intelligence and knowledge. This latter property Diogenes proved by the succession of climatic and atmospheric phenomena of winter and summer, night and day, rain, wind, and fine weather. All these successions were disposed in the best possible manner by the air: which could not have laid out things in such regular order and measure, unless it had been endowed with intelligence. Moreover, air was the source of life, soul, and intelligence, to men and animals: who inhaled all these by respiration, and lost all of them as soon as they ceased to respire.[184]
[184] Diog. Apoll. Fr. iv.-vi. c. 36-42, Panz. — Οὐ γὰρ ἂν οὕτω δέδασθαι οἷόν τε ἦν ἄνευ νοήσιος, ὥστε πάντων μέτρα ἔχειν, χειμῶνός τε καὶ θέρεος και νυκτὸς καὶ ἡμέρης καὶ ὑετῶν καὶ ἀνέμων καὶ εὐδιῶν. καὶ τὰ ἄλλα εἴ τις βούλεται ἐννοέεσθαι, εὕρισκοι ἂν οὕτω διακείμενα, ὡς ἀνυστὸν κάλλιστα. Ἔτι δε πρὸς τούτοις καὶ τάδε μεγάλα σημεῖα· ἄνθρωπος γὰρ καὶ τὰ ἄλλα ζῶα ἀναπνέοντα ζώει τῷ ἀέρι. Καὶ τοῦτο αὐτοῖς καὶ ψυχή ἐστι καὶ νόησις ——
— Καὶ μοὶ δοκέει τὸ τὴν νόησιν ἔχον εἶναι ὁ ἀὴρ καλεόμενος ὑπὸ τῶν ἀνθρώπων, &c.
Schleiermacher has an instructive commentary upon these fragments of the Apolloniate Diogenes (Vermischte Schriften, vol. ii. p. 157-162; Ueber Diogenes von Apollonia).