HaRav Shneur Zalman MiliadiRav Shneur Zalman (September 4, 1745 – December 15, 1812) of Liadi, was the founder of Chabad hassidism and the author of many works, among them Shulchan Aruch HaRav, Tanya and the Siiddur Torah Or . Chabad became a worldwide Jewish spiritual outreach movement.
וכמו שכתוב גם כן במקום אחר משל גשמי לזה, מענין ביטול זיו ואור השמש במקורו, הוא גוף כדור השמש שברקיע
Elsewhere, this idea is further illustrated by an analogy from a physical phenomenon — the nullification of the sun’s radiance and light within its source, the celestial orb of the sun.
שגם שם מאיר ומתפשט ודאי זיוו ואורו, וביתר שאת מהתפשטותו והארתו בחלל העולם, אלא ששם הוא בטל במציאות במקורו, וכאילו אינו במציאות כלל
For surely its radiance and light glow and spread forth there too; more strongly, in fact, than they spread forth and glow in the space of the universe. Being close to its source, the light is more intense. But there — within the sun — its very existence is nullified within that of its source; it is as though [the light] were absolutely non-existent. All that is seen within the sun is the sun itself, not the light which is merely a product, an offshoot of the sun.
This will be better understood in terms of the saying,2 “Of what good is a candle in the daylight” Naturally, the candle is no less luminous by day than by night. But because its light is overwhelmed by the far greater brightness of daylight, it no longer fulfills its function of illumination. At this point it ceases to exist as a luminary. The same is true of the sun’s rays as they are within the sun.
וככה ממש דרך משל הוא ביטול העולם ומלואו במציאות לגבי מקורו, שהוא אור אין סוף ברוך הוא, וכמו שכתוב שם באריכות
Exactly so, figuratively speaking, is the very existence of the world and everything in it nullified in relation to its source, which is the light of Ein Sof, as is explained there at length.
This, then, is the true meaning of G‑d’s unity — that He alone exists, and there is nothing besides Him.
2. Chullin 60b; cf. Zohar I, 20a.