The phrase that way madness lies and its variants mean that a proposed course of action is bound to lead to disaster.
This phrase refers to the following from M. William Shak-speare: His True Chronicle Historie of the life and death of King Lear and his three Daughters (London: Printed for Nathaniel Butter, 1608), by the English poet and playwright William Shakespeare (1564-1616)—Lear shies away from contemplating the ingratitude of his daughters, Regan and Goneril:
O Regan, Gonorill, your old kind father
Whose franke heart gaue you all, O that way madnes lies,
Let me shun that, no more of that.
These are, in chronological order, the earliest occurrences that I have found of the phrase that way madness lies—without explicit reference to Shakespeare’s King Lear:
1-: From the unsigned review of The History of Ancient Wiltshire (London: W. Miller, 1810), by Richard Colt Hoare (1758-1838)—review published in The Quarterly Review (London, England) of February 1811:
Let him dig, delineate, describe, engrave, (hæ tibi erunt artes *,) but beware of theory, ‘for that way madness lies.’
* This is a quotation from the Aeneid (Book VI, lines 851-853), by the Roman poet Virgil (Publius Vergilius Maro – 70-19 BC)—cf. details here:
tu regere imperio populos, Romane, memento;
hae tibi erunt artes; pacisque imponere morem,
parcere subiectis, et debellare superbos.
You, O Roman, govern the nations with your power—remember this!
These will be your arts—to impose the ways of peace,
To show mercy to the conquered and to subdue the proud.
2-: From the concluding lines of a poem titled Farewell to Snuff. An Elegy, published in the Cornwall Gazette, Falmouth Packet & Plymouth Journal (Truro, Cornwall, England) of Saturday 12th October 1811:
Snuff and Science, are to me a dream;—
But hold, my Heart! for that way madness lies?
Love’s in the scale—Tobacco kicks the beam.
3-: From a letter that the Irish nationalist leader and social reformer Daniel O’Connell (1775-1847) wrote from Dublin on Monday 4th January 1813 to the Editor of The Examiner (London, England)—published in that newspaper on Sunday 24th January 1813:
Englishmen—most thinking Englishmen—[…] are now so convinced that Irish Papists entertain opinions as subversive of morality as they are inconsistent with common sense—that they—most thinking Englishmen—discredit and reject the plighted faith—the solemn oath—the unstained honour of the high-minded ancient gentry of Ireland—the last relic, alas! of the sainted chivalry of Europe.
Strange perversion of doating credulity! You believe us to be perjurers—but so absurd in our perjury, as not to swear to that falshood [sic] which would be useful to us—whilst we readily swear to an useless lie.—Sir, I could submit—as I do submit, to oppression—but I know not how to brook the absurd insult offered as a justification of that oppression—that way indeed madness lies—I therefore turn from it.
There is an echo of the phrase in the following from The Bride of Abydos. A Turkish Tale (London: Printed by T. Davison, 1813), by the English poet George Gordon Byron (1788-1824):
And, oh! that pang where more than Madness lies—
The Worm that will not sleep—and never dies—
Thought of the gloomy day and ghastly night,
That dreads the darkness, and yet loathes the light—
That winds around, and tears the quiv’ring heart—
Ah! wherefore not consume it—and depart!