Yih-sin, variously spelled as "yin sin", "yinsin" or "yi-hsin", is a term found in The Secret Doctrine and in The Mahatma Letters. The term, claimed to be Chinese in origin, cannot be found in the given spelling. David Reigle has proposed that it is a phonetic spelling of the term yixin in the pinyin system, meaning "the one mind". Another clue could be in the reversed term sin-yin, which means the "heart's seal" of the Buddha.
Yih-sin is mentioned in two of The Mahatma Letters, and translated following Samuel Beal's definition (see below) as the "Universally diffused essence" and the "one form of existence" (this latter translation should not be confused with the phrase "the one form of existence" occurring in verse 8 of stanza I of the The Secret Doctrine, which Blavatsky said was a translation of the Sanskrit word prabhavāpyaya.)
This 'force' so-called, shows itself truly indestructible but does not correlate and is not convertible in the sense accepted by the Fellows of the R.S., but rather may be said to grow and expand into 'something else' while neither its own potentiality nor being are in the least affected by the transformation. Nor can it well be called force since the latter is but the attribute of Yin Sin (Yin Sin or the one 'Form of existence' also Adi-Buddhi or Dharmakaya the mystic, universally diffused essence) when manifesting in the phenomenal world of senses namely only your old acquaintance Fohat.
A similar reference is found in Letter No. 111 although here Yih-sin is said to be "the child of Dharmakāya":
In symbology the central point is Jivatma (the 7th principle), and hence Avalokitesvara, the Kwan-Shai-yin, the manifested 'Voice' (or Logos), the germ point of manifested activity; -- hence -- in the phraseology of the Christian Kabalists 'the Son of the Father and Mother,' and agreeably to ours -- 'the Self manifested in Self' -- Yih-sin, the 'one form of existence,' the child of Dharmakaya (the universally diffused Essence), both male and female. Parabrahm or 'Adi-Buddha' while acting through that germ point outwardly as an active force, reacts from the circumference inwardly as the Supreme but latent Potency.
Helena Petrovna Blavatsky mentions "Yin-Sin" in connection with absolute consciousness:
The true Buddhist, recognising no “personal god,” nor any “Father” and “Creator of Heaven and Earth,” still believes in an absolute consciousness, “Adi-Buddhi”; and the Buddhist philosopher knows that there are Planetary Spirits, the “Dhyan Chohans.” But though he admits of “spiritual lives,” yet, as they are temporary in eternity, even they, according to his philosophy, are “the maya of the day,” the illusion of a “day of Brahmâ,” a short manvantara of 4,320,000,000 years. The “Yin-Sin” is not for the speculations of men, for the Lord Buddha has strongly prohibited all such inquiry.
Thus, were one to translate into English, using only the substantives and technical terms as employed in one of the Tibetan and Senzar versions, Verse I would read as follows: — 'Tho-ag in Zhi-gyu slept seven Khorlo. Zodmanas zhiba. All Nyug bosom. Konch-hog not; Thyan-Kam not; Lha-Chohan not; Tenbrel Chugnyi not; Dharmakaya ceased; Tgenchang not become; Barnang and Ssa in Ngovonyidj; alone Tho-og Yinsin in night of Sun-chan and Yong-grub (Parinishpanna), &c., &c.,' which would sound like pure Abracadabra.
Identification of the term
The term "yih-sin" appears in the book A Catena of Buddhist Scriptures from the Chinese, written by the scholar Samuel Beal in 1871:
So again, when the idea of a universally diffused essence (dharmakaya) was accepted as a dogmatic necessity, a further question arose as to the relation which this 'supreme existence' bore to time, space, and number. And from this consideration appears to have proceeded the further invention of the several names Vairochana (the Omnipresent), Amitabha (for Amirta) the Eternal, and Adi-Buddha (yih-sin) the 'one form of existence.'
David Reigle proposes that this term used by Beal was his phonetic transcription of yixin (一心, yīxīn), meaning "one-mind":
In the word "Yinsin" the [first] "n" should be "h". It is found in The Mahatma Letters, 2nd ed., letter #15 as: "Yin Sin or the one 'Form of Existence'," and in letter #59 as: "Yih-sin, the 'one form of existence'." It is found in Samuel Beal's 1871 book, A Catena of Buddhist Scriptures from the Chinese, p. 373, as: "(yih-sin) the 'one form of existence'." Beal's early phonetic transcription "yih-sin" is in the later Wade-Giles system of transcription "i-hsin," and in the current pinyin system of transcription "yixin." It means the "one (yih, i, yi) mind (sin, hsin, xin)," Sanskrit eka-citta. It is what the Awakening of Faith starts with at the opening of its first chapter. It is the "True Mind" of Fa-tsang's commentary.
An alternative source could be the reversed form sin-yin (心印, xīn-yìn) meaning the "heart's seal," which is said to contain "the whole mind of Buddha."  According to J. Edkins, this term is associated to the swastika placed on the Buddha's chest, symbolizing his esoteric doctrines.
- The One Mind by David Reigle at PrajnaQuest.fr
- Vicente Hao Chin, Jr., The Mahatma Letters to A.P. Sinnett in chronological sequence No. 67 (Quezon City: Theosophical Publishing House, 1993), 182.
- Vicente Hao Chin, Jr., The Mahatma Letters to A.P. Sinnett in chronological sequence No. 111 (Quezon City: Theosophical Publishing House, 1993), 378-379.
- Helena Petrovna Blavatsky, The Secret Doctrine vol. I, (Wheaton, IL: Theosophical Publishing House, 1993), 635.
- Helena Petrovna Blavatsky, The Secret Doctrine vol. I, (Wheaton, IL: Theosophical Publishing House, 1993), 23.
- Samuel Beal, A Catena of Buddhist Scriptures from the Chinese (London: Trubner & Co., 1871), 373.
- David Reigle at Theosophy Nexus forum
- Joseph Edkins, Chinese Buddhism by (London: Trubner and Co., 1880), 63.