@EquesLignite,
In the context of the passage in which Aristotle is discussing wise precautions against revolution, he appears to be warning against the sudden increase in honour or fame, power, or excessive wealth. It is to monitor these, it seems, that he suggests an impartial magistry. In the sentences immediately following the quotation in question, Aristotle argues that to avoid these evils, it is necessary to "give the management of affairs and offices of the state to opposite elements;" this "balance of powers" will, he writes, put an end to revolutions caused by excessive power whether derived from too much money or through influence. Nothing in the text suggests Aristotle envisoned the need for a large secret police overseen by some council. I think we have to remind ourselves that Aristotle's political thinking in this treatise centered around the polis, or city-state, typical in Greece; the citizen population of Athens in Aristotle's time is estimated to be around 40,000. In this kind of "small-town" atmosphere, an extensive spy service of the type in Communist Poland would have been unnecessary.
Regards, and thanks for providing the citation.
John