Truth is one, not a second; indeed; non-truth shines forth like truth;
The Siva Linga is just a stone, not a second made by a sculptor.
Commentary
In this concluding verse the sage tells us that "Truth is one, not a second", and that even "non-truth shines forth as truth" which means that non-truth is part of truth, enveloped by truth, so to say, because truth does not have a second. If truth had a second it would not be absolute. What the sage has declared here in the first line, the Brihadaranyaka Upanishad (V 5.1) tersely explains, "Those Gods adored Truth, that (Truth) is of three syllables sa, ti, yam,— sa is one syllable, ti is one syllable, yam is one syllable. The first and last syllables are Truth. In the middle is the untrue. This untrue is embraced on both sides by Truth; it partakes of the nature of Truth itself. The untrue does not harm him who knows this."
Although this chapter is titled "Vision of Non-Truth" the sage uses non-truth to enlighten us as to what is Truth. The Absolute is truth, and embodied in Truth are the relative 'true' and 'false'. Understanding the false is a form of truth. Saying one is ignorant is also a form of truth. It is when the false shines forth as truth, and the person accepts it, that there is confusion and duality. The Chandogya Upanishad (VII. 26.2) describes the person of yogic vision in this way: "The sage sees only the totality, completely obtaining the totality."
In the opening verse of the next chapter, Narayana Guru describes illusive relativity as shining forth as knowledge, non-knowledge, etc. When the sage here says: Truth is one, not a second", he is making the non-dual Absolute its ground, as the Absolute which pours out through the agency of illusive relativity the confusing pairs of opposites. In the Siva Sutra (I.2) it is said, "Wisdom is bondage." In the commentary (Siva Sutra Varttika) one reads this elucidation on the above verse: "This wisdom which has the modifications of feeling inwardly happiness, etc. tends outwardly towards sense objects and causes duality for the person, and is called bondage." It is the wisdom of the world meant for those who are attached to the world and have no desire for spiritual freedom.
In the second line the sage says, "the Siva Linga is just a stone", and so he rejects any secondary image or structure made by a sculptor. Truth is right before us, inside us, and we need not look anywhere else. As the Rig Veda (VII. 104.12) says, "Truth and non-truth are opposites; of these two, truth is that which is direct." What is directly before us is there. When our concept making minds run riot and interfere there is confusion. In the words of the Yoga Sutra (I.9) "Mental fancy is a concept conveyed by mere words of which there is no object corresponding to reality."