This is from Aristotle’s metaphysics – although I have just read it in “texts to illustrate the history of Greek philosophy” in the chapter “Ionian henists”. I have only seen the word henist in this book so I suspect that the compiler made it up to describe those who thought there was only one element e.g. water or earth.
τῶν δὴ πρώτων φιλοσοφησάντων οἱ πλεῖστοι τὰς ἐν ὕλης εἴδει μόνας ᾠήθησαν ἀρχὰς εἶναι πάντων: ἐξ οὗ γὰρ ἔστιν ἅπαντα τὰ ὄντα καὶ ἐξ οὗ γίγνεται πρώτου καὶ εἰς ὃ φθείρεται τελευταῖον, τῆς μὲν οὐσίας ὑπομενούσης τοῖς δὲ πάθεσι μεταβαλλούσης, τοῦτο στοιχεῖον καὶ ταύτην ἀρχήν φασιν εἶναι τῶν ὄντων, καὶ διὰ τοῦτο οὔτε γίγνεσθαι οὐθὲν οἴονται οὔτε ἀπόλλυσθαι, ὡς τῆς τοιαύτης φύσεως ἀεὶ σωζομένης, ὥσπερ οὐδὲ τὸν Σωκράτην φαμὲν οὔτε γίγνεσθαι ἁπλῶς ὅταν γίγνηται καλὸς ἢ μουσικὸς οὔτε ἀπόλλυσθαι ὅταν ἀποβάλλῃ ταύτας τὰς ἕξεις, διὰ τὸ ὑπομένειν τὸ ὑποκείμενον τὸν Σωκράτην αὐτόν, οὕτως οὐδὲ τῶν ἄλλων οὐδέν: ἀεὶ γὰρ εἶναί τινα φύσιν ἢ μίαν ἢ πλείους μιᾶς ἐξ ὧν γίγνεται τἆλλα σωζομένης ἐκείνης.
Most of the first people to practise philosophy thought that it was only material substances that were the origin of everything. All things are made of this and all things are generated from this and all things are destroyed at the end into this; the underlying essence always remain but the attributes change. This is the basic element and this is the origin of things that exist. Because of this they think that nothing is created or destroyed as its essential nature is always preserved. For example we don’t say that Socrates comes into being when he becomes beautiful or musical nor that he is destroyed when he loses these attributes; because Socrates himself is the thing that is constant and likewise with everything else. For there is always some natural substance (or more than one) from which everything else is generated while it itself is preserved.
Aristotle Metaphysics A 3 983 b 7