Yàn Sīshèng 晏斯盛, zì Yīzhāi 一齋, was a Kāngxī-Yōngzhèng-Qiánlóng-period official and Yìjīng scholar from Xīnyú 新喻 (Yuánzhōu 袁州, Jiāngxī 江西). He passed the jìnshì in Kāngxī xīnchǒu 康熙辛丑 = 1721 and held office through Húběi 湖北 Governor (Húběi xúnfǔ 湖北巡撫). The catalog meta provides no birth-and-death dates.
His Yì-corpus comprises three works grouped together in the Sìkù under his name: the Xué Yì chū jīn 學易初津 (KR1a0147) in two juàn (the methodological framework); the Yì yì zōng 易翼宗 in six juàn (the commentary on the canonical scripture, integrating Wing material under the canonical text); and the Yì yì shuō 易翼說 in eight juàn (commentary on the Ten Wings). Together they form a programmatic mid-Qiānlóng Yìxué statement.
Yàn’s methodology is a moderate compromise: he follows the canonical text closely; rejects the HétúLuòshū binary doctrine (the so-called “transmitted charts” he says are Dàyǎn 大衍 derivative numbers extracted from the Dàzhuàn); rejects guà biàn and hùtǐ; rejects fāngjì shùshù speculation; rejects pure metaphysical “principle-and-breath, mind-and-nature” talk. The Sìkù editors describe his position as “still substantial-and-near-reason among recent Yì-houses.”