In King Lear, Lear’s first major mistake comes in Act 1, Scene 1, when he sternly rebukes his daughter Cordelia for failing to flatter him with a proclamation of her love. He can scarcely believe his ears when Cordelia refuses, again and again, to participate in the competition for her father’s love:
Lear: Nothing will come of nothing. Speak again.
Cordelia: Unhappy that I am, I cannot heave
My heart into my mouth. I love your majesty
According to my bond, no more nor less.
When Lear demands that Cordelia make another attempt to lavish him with praise, he alludes to a famous phrase from Classical philosophy, first attributed to the Greek philosopher Parmenides: nothing comes from nothing. Conversely, for anything to exist, it must come from something. This idea was important to the major philosophers of antiquity (like Parmenides himself and, later, Lucretius), who sought to explore the origins of the universe itself. They wanted to know how things come into being and stay that way, and such a concept would no doubt be on Lear's own mind as he ponders the best way to ensure the continuation of his kingdom. It's worth pointing out that Parmenides was alive several hundred years after the events of King Lear, but this doesn't change the fact that Shakespeare (and, perhaps, his audiences) must have been familiar with his ideas.
In Act 2, Scene 2, as Cornwall and Kent verbally spar, Cornwall accuses Kent of being too blunt with his words. Kent responds in flowery language, embellishing his point with an elaborate simile to mock Cornwall:
Kent: Sir, in good faith, or in sincere verity,
Under th' allowance of your great aspect,
Whose influence, like the wreath of radiant fire
On flickering Phoebus' front—
Cornwall: What mean’st by this?
Kent: To go out of my dialect, which you discommend so much.
I know, sir, I am no flatterer. He that beguiled you in
A plain accent was a plain knave, which for my part I
Will not be, though I should win your displeasure to
Entreat me to ‘t.
Kent’s simile compares Cornwall's face, which Kent terms his “great aspect,” to the glowing forehead of Phoebus—also known as Apollo—the Greek god of the sun. Kent’s use of a classical allusion and a simile throws Cornwall off, as Kent has spent the first part of the conversation speaking in rude and direct observations. Despite himself, his sarcasm drips through what might otherwise seem like an elaborate compliment; “great aspect” may also be taken to refer to the size of Cornwall’s forehead. Only a “plain knave” would try to trick Cornwall in a “plain accent,” Kent assures, as he switches to a more sophisticated register and continues his insults.
In Act 3, Scene 2, the audience is treated to a dramatic prophecy offered by King Lear’s court Fool. As is typical for a Shakespearean fool by virtue of his outcast status in the court and his whimsical demeanor, the Fool is able to cut straight to the truth and even see directly into the future without fear of reprisal from Lear or other nobility. In this passage, he boldly alludes to an old legend:
[...] I’ll speak a prophecy ere I go:
When priests are more in word than matter,
When brewers mar their malt with water,
When usurers tell their gold i’ th’ field,
And bawds and whores do churches build—
Then shall the realm of Albion
Come to great confusion;
Then comes the time, who lives to see ‘t,
That going shall be used with feet.
This prophecy Merlin shall make, for I live before his time.
As the Fool ends his prophecy, he alludes to the character Merlin from Arthurian legend. Now popularized as a powerful wizard, Merlin—also known in early legends from the period of Anglo-Saxon control of England as Myrddin—was a prophet who delivered his prophecies in verse not unlike the example above. By including an allusion to a figure not introduced into the world of British mythology until after the time in which King Lear is set, Shakespeare reminds the audience of the early setting of his play; he also gives his Fool particular credibility by showing him predict such a widely recognizable figure.
Throughout his feigned madness as Poor Tom, Edgar mutters a number of strange and demonic names. In Act 3, Scene 4, for instance, he says:
This is the foul fiend Flibbertigibbet. He begins at curfew and walks till the first cock. He gives the web and the pin, squints the eyes, and makes the harelip, mildews the white wheat, and hurts the poor creature of the earth.
This accursed name-dropping continues later in the same scene:
The Prince of Darkness is a gentleman. Modo he's called, and Mahu.
Shakespeare appears to have drawn on a book by Samuel Harsnett, the Declaration of Egregious Popish Impostures, published just a few years before Shakespeare wrote King Lear. Harsnett was a noted writer in his time and later served as the Archbishop of York. In Declaration, a group of Jesuits exorcise somebody named "Flibbertigibbet" (the word "flibbertigibbet" is now used to refer to overly silly, chatty, unserious people). The Declaration features—in addition to the above names and phrases—a critique of Roman Catholic exorcism practices.
The use of this language in King Lear shows an awareness of contemporary religious debates that may have been popular with Shakespeare's original audiences, although the use of Christian writing in a play set in pre-Christian Britain is a curious anachronism. More likely than not, Harsnett's documentation of arcane demonic names and superstitions provided ample fodder for the sort of "mad-speak" that makes Shakespeare's portrayals of insanity at once whimsical and convincing—after all, the name "Flibbertigibbet" sounds, in and of itself, completely insane.
In the chaos of the storm on the heath, Lear’s madness is matched only by the pretend madness of “Poor Tom”—Gloucester’s son Edgar in disguise. The crazed Lear grows something of an attachment to Poor Tom and professes to hold the man in high esteem:
Kent: Good my lord, take his offer; go into th' house.
Lear: I'll talk a word with this same learnèd Theban.—
What is your study?
With him! I will keep still with my philosopher.
Kent [to Gloucester]: Good my lord, soothe him. Let him take the fellow.
Gloucester [to Kent]: Take him you on.
Kent [to Edgar]: Sirrah, come on. Go along with us.
Lear: Come, good Athenian.
In Lear’s eyes, Edgar is as formidable a mind as the legendary thinkers of Ancient Greece; their interactions are rife with allusions to classical city-states like Thebes and Athens, once known for producing many famous philosophers. What's interesting, though, is that this allusion is somewhat anachronistic, given that King Lear is set at some point in the 8th Century BCE—that is, some 200 hundred years before the period generally associated with the most famous Ancient Greek philosophers. Nonetheless, Shakespeare's audience would have recognized that phrases like "learnèd Theban" and "good Athenian" referred to the celebrated thinkers of Ancient Greece, even if these thinkers hadn't yet come to prominence in the actual world of the play.
Lear’s insistence of Edgar’s formidable intelligence highlights their shared bond of insanity and highlights a common trope of Shakespeare’s plays: in the midst of a veritable battlefield of conflicting agendas, ambitions, and constant back-stabbing, it is often the madmen and the fools who—like Socrates and Aristotle themselves—are able to identify and share the real truth.
Whereas King Lear faces a genuine descent into raving madness, Edgar’s adopted persona of Poor Tom is only feigned insanity. At times, Edgar leans too far into his imitation and comes across—to humorous effect—as entirely nonsensical, as in this sequence from Act 3, Scene 4, which includes two strange allusions:
Lear: Come, good Athenian.
Gloucester: No words, no words. Hush.
Edgar: Child Rowland to the dark tower came.
His word was still "Fie, foh, and fum,
I smell the blood of a British man."
As is often the case for the mad characters in King Lear, a mark of “Poor Tom’s” insanity comes from his reliance on unrelated and jumbled allusions. In this passage, he makes two simultaneously. First, Edgar references Roland, a historical knight in Charlemagne’s court who grew into a figure of French legend in the Old French epic The Chanson de Roland, or The Song of Roland. Second, he quotes the catchphrase of the giant in the English fairy tale of Jack and the Beanstalk. There is not much that can be made of these allusions, other than that they display a typical Shakespearian awareness of European folklore and would have been readily recognizable to an English audience. In a way, this almost makes Edgar's raving seem like it has substance, but the fact that the references don't really connect to any broader point ultimately emphasizes his (feigned) insanity. The blending of French and English stories in Edgar’s blathering could also be said to reflect the broader tensions between the French and English in the play, as well as the positioning of the impending French invasion as a major external plot point. More than anything, though, the effect of these allusions is to show Edgar’s efforts, as Poor Tom, to sustain his nonsensical rambling.