A demagogue, from the Greek δημαγωγoς (dēmagōgos), is the leader or agogos of the dēmos. Today, we tend to associate the dēmos with the people. And this association is usually taken positively. Take for instance the term democracy [dēmos plus kratos (here, rule but can also mean power/strength)]. But in classical Greece where the demagogue emerged, the dēmos meant more like the mob. Or more precisely, the common people. And the common people did not constitute a group that attracted positive valuation.
In fact, the term “vulgar” which has all sorts of negative connotations means “common” and was until quite recently frequently used to refer to the common or ordinary, i.e. uneducated, public. (It is of course no longer used in that sense.) Consider the following statement by the Scottish philosopher David Hume from his Dialogues Concerning Natural Religion .
The vulgar, indeed, we may remark, who are unacquainted with science and profound enquiry, observing the endless disputes of the learned, have commonly a thorough contempt for philosophy; and rivet themselves the faster, by that means, in the great points of theology, which have been taught them.Part 1, paragraph 3.
To the extent that the demagogue appealed to this dēmos — this uncultured, uneducated mob, the commoners — by engaging their passions through fantastic speech and fallacious reasoning, he not only attracted censure from philosophers and but was as seen as dangerous for the polis or the city.
— Lucky, lucky man,186–193, trans. Benjamin Bickley Rogers.
what a start you’ve got for public life.
— But I know nothing, friend, beyond my letters,
And even of them but little, and that badly.
— The mischief is that you know ANYTHING.
To be a Demus-leader [δημαγωγία (dēmagōgia)] is not now
For lettered men, nor yet for honest men,
But for the base and ignorant.
Another example is Thucydides. In his History of the Peloponnesian War [composed throughout the war, and unfinished by the time of his death c. 400 BCE?], Thucydides writes (and this again is about Cleon):
But the Athenians believed that, since they [the Athenians] held the men [the Spartans] on the island [of Sphacteria near Pylos], peace could be theirs [the Athenians] the moment they cared to make it, and meanwhile they were greedy for more. They were urged to this course chiefly by Cleon son of Cleaenetus, a popular leader [δημαγωγος (dēmagōgos)] at that time who had very great influence with the multitude.4.21.2–3, trans. Charles Forster Smith.
This was in 425 BCE. Athens had the advantage then. Sparta pleaded for peace but was repudiated by the Athenians at the instigation of Cleon, according to Thucydides at least. This would later prove to be disastrous for the Athens and for Cleon himself who was killed in 422 BCE.
Of course, there is so much more to the demagogue than what Aristophanes and Thucydides say. But their representations have stuck. And when the term is invoked to describe a politician, it is almost always negative.
Aristophanes, Knights, in The Comedies of Aristophanes, vol 1, edited, translated, and explained by Benjamin Bickley Rogers (London: George Bell, 1910).
David Hume, Dialogues Concerning Natural Religion and Other Writings, edited by Dorothy Coleman, Cambridge Texts in the History of Philosophy (Cambridge: Cambridge University Press, 2007).
Thucydides, History of the Peloponnesian War, translated by Charles Forster Smith, 4 vols., Loeb Classical Library (Cambridge, MA: Harvard University Press [first print: 1919–1923]).