12.3 ये who, तु verily, अक्षरम् the imperishable, अनिर्देश्यम् the indefinable, अव्यक्तम् the unmanifested, पर्युपासते worship, सर्वत्रगम् the omnipresent, अचिन्त्यम् the unthinkable, च and, कूटस्थम् the unchangeable, अचलम् the immovable, ध्रुवम् the eternal.

Commentary:
Anirdesyam That which cannot be actually shown or which cannot be defined -- the Akshaa or Satchidananda Para Brahman is beyond the reach of the mind and speech. Why can It not be defined Because It is unmanifested. It does not have the four alities of manifested beings, vi.z, Jati (caste such as Brahmana, Kshatriya, etc.), Guna (attributes such as blueness, whiteness, tallness, shortness, etc.), Kriya (reading, walking, etc.), and Sambandha (like the relation between father and son).The unmanifest Incomprehensible by any of the organs of knowledge not manifest to any of the organs of knowledge.Upasana (worship) means sitting near. It is approaching the chosen ideal or object of worship by meditating on it, in accordance with the teachings of the scriptures and the spiritual preceptor, and dwelling steadily in the current of that one thought like a threat of oil poured from one vessel to another. It means continous and uninterrupted contemplation of God.The imperishable Brahman is omnipresent, pervading everything like the ether. It is unthinkable, because It is unmanifest. Whatever is visible to the senses can be thought of by the mind also. That which can be grasped by the organs of knowledge can be thought of by the mind also. But the Supreme Being is invisible to the senses and so cannot be grasped by the organs of knowledge and is, therefore, unthinkable. All thoughts of God ultimately lead the aspirant to iescent meditation.It is Kutastha (unchangeable). Kutastha means remaining like a mass or a heap. Therefore It is immutable and eternal. Just as the anvil remains unchanged though the ironpieces which are beaten on the anvil change their shape, so also Brahman is unchanging though the forms are changing. Hence Brahman is called Kutastha. Kuta also means a thing which appears to be good externally but which is full of evil within. Hence it refers to that seed of Samsara, viz., ignorance, which is full of evil within and which is known as the Avyakrita (undifferentiated) in the Svetasvataropanishad (Mayam tu prakritim vidyat, Mayinam tu mahesvaram) and in the Gita (Mama maya duratyaya -- The illusion of Mine is hard to pierce -- VII.14). Another interpretation for Kutastha is that which is at the root of everything. He Who is seated in Maya as its witness, as its Lord, is Kutastha.Achalam Immovable, that which is free from change. Therefore the imperishable Brahman is Dhruvam, eternal. (Cf.VIII.21)