Galen’s Simple Drugs 8 preface

The remaining entries on the capacity in plants will be discussed in this [book], not every capacity, but only the activities (energeiai) of the active qualities (drastikai poiotētes). For I will examine the functions of plants’ whole substance on their own at a later time. As a reminder again for those who have read the previous [books], I have organized each simple capacity into four degrees based on the extent to which they act more or less. I defined the scope of each order with clear boundaries and taught a certain method for discovering the capacities themselves. I produced demonstrations for all these [categories] and set up objectives and criteria for each capacity, both specific and common. I will thus now add what remains, urging that if anyone is not acquainted with the previous [books], either to completely avoid these [topics] or, if they are diligent, to first go back and read the treatise from the beginning. For all the topics to be discussed will appear to them unclear and undemonstrated, until they have trained themselves in those [previous discussions].

Ὅσον μὲν οὖν ὑπόλοιπόν ἐστι τῆς κατὰ φυτὰ δυνάμεως ἐν τῷδε λεχθήσεται, οὐδὲ τοῦτο ἅπαν, ἀλλ' αἱ τῶν δραστικῶν ποιοτήτων ἐνέργειαι μόναι. τὰ γάρ ὅλης ἔργα τῆς οὐσίας τῶν φυτῶν ὕστερον αὐτὰ καθ' αὑτὰ διέξιμεν. ἀναμνήσαντες οὖν πάλιν κατ' αὐτὰ τοὺς ἀνεγνωκότας τὰ πρόσθεν, ὡς καθ' ἑκάστην δύναμιν ἁπλῆν εἰς τάξεις τέτταρας ἐποιησάμεθα τὴν ἐν τῷ μᾶλλόν τε καὶ ἧττον διαίρεσιν, ὅτι τε σαφέσιν ὅροις ἑκάστης τάξεως τὸ πλάτος ἀφωρισάμεθα καὶ αὐτῆς τῶν δυνάμεων εὑρέσεως ἐδιδάξαμέν τινα μέθοδον, ἁπάντων τε τούτων ἀποδείξεις ἐποιησάμεθα καὶ σκοποὺς ἐθέμεθα καὶ κριτήρια καθ' ἑκάστην δύναμιν ἴδιὰ τε καὶ κοινὰ, προσθήσομεν οὕτως ἤδη τὰ λείποντα, παρακαλέσαντες, εἴ τις οὐχ ὡμίλησε τοῖς ἔμπροσθεν, ἤτοι καὶ τούτων ἀφίστασθαι παντάπασιν, ἢ εἴπερ ἦν φιλόπονος ἐξ ἀρχῆς πρότερον ἀναλέξασθαι τὴν πραγματείαν. ἀσαφῆ τε γάρ ἅμα καὶ ἀναπόδεικτα πάντ' αὐτῷ φανεῖται τὰ λεχθησόμενα, πρὶν ἐν ἐκείνοις γυμνάσασθαι.

Galen, On the Capacities of Simple Drugs, VIII. proem, 12.83–84 K.