The provocation
In his 1966 doctoral dissertation, American Korean-studies scholar Gari Ledyard argued that King Sejong's Hunminjeongeum (訓民正音, 1443) did not arise ex nihilo from articulatory phonetics, as the official 1446 explanatory text claims. Rather, Ledyard proposed, Sejong silently adapted the Phags-pa script — the universal Yuan-imperial alphabet designed in 1269 by Drogön Chögyal Phagpa — for five of Hangul's basic consonant shapes.
The evidence
Ledyard pointed to visual parallels in five letters: ㄱ, ㄷ, ㄹ, ㅂ, ㅅ, each resembling a corresponding Phags-pa consonant (ꡂ, ꡊ, ꡙ, ꡎ, ꡛ). He noted that Phags-pa was no dead letter in Korea: Goryeo had been under Mongol rule when the script was new, and Joseon still ran official courses in it — instruction was in decline by 1423, but had not stopped. By 1443, Phags-pa was already two generations dead, but its forms survived in the books the Korean court kept, including Sino-Mongol rhyme material of the Menggu Ziyun type that Sejong's philologists worked from.
The rebuttal
Korean linguists, notably Kim Wanjin and Lee Ki-Moon, replied that the Hunminjeongeum Haerye (1446) explicitly describes Hangul consonant shapes as iconic representations of the articulatory organs: ㄱ as the back of the tongue raised, ㄴ as the tongue touching the alveolar ridge, ㅁ as the mouth, etc. This articulatory theory has no parallel in any other script; if Hangul were borrowed, why would such an elaborate cover story be invented?
No settled verdict
Western scholarship is split rather than settled. Peter T. Daniels accepts Ledyard's derivations; Richard Sproat and Jae Jung Song call the influence possible but unproven; Ross King (1996) finds none of the inspiration theories more convincing than independent invention, and Geoffrey Sampson (2015) treats Hangul's design as an original featural system. What is not disputed is that the theoretical framework — the articulatory phonetics — is Sejong's own. The deeper question has shifted from 'did he copy' to 'how much can total originality really exist'.