Cause & effect relationship

Every limited name and form is endowed with a limitless nature. This is because brahman, which is the source of this entire manifestation, alone is limitless; and the effect is not different from the cause. Just as cloth has cotton-ness because it is an effect of cotton, the universe derives its limitless nature from brahman alone.

With the desire to ascertain this fact, the last section of the Katha upanisad begins by comparing the universe to a peepal tree (aśvattha) and brahman to its root.

ūrdhvamūlo’vākśākha eṣo'śvatthaḥ sanātanaḥ |

tadeva śukram tadbrahma tadevāmṛtamucyate |

tasmillokāḥ śritāḥ sarve tadu nātyeti kaścana |

etadvai tat ||2.3.1||

This (universe) is like an aśvattha tree with its root above and branches below, remaining permanent. That root alone is pure (shining); it alone is brahman and is called immortal. In it alone, all worlds are contained, and nothing indeed transcends it. This is indeed that.

The first line of the mantra describes this samsāra world as aśvatthaḥ - peepal tree. The entire manifestation is compared to a peepal tree because of its vastness and long-lasting nature. Thus, it is described as sanātanaḥ - permanent, not permanently unchanging, but continuously recurring, keeping recycling without end. This world is seen as a tree that has its roots above - ūrdhvamūlaḥ, whose origin (brahman) is of a higher nature. The root is not visible because that origin is beyond the regular means of knowledge. This tree has branches extending downward - avākśākhaḥ, representing the visible manifested world and its lower nature. That root alone shines as consciousness - tadeva śukram. That alone is brahman- tadbrahma. tadevāmṛtamucyate - that alone is called immortal (limitless) in the śāstra. tasmillokāḥ śritāḥ sarve - all the worlds (the fourteen spheres of experience) have brahman as their support, everything exists dependent upon brahman. tadu nātyeti kaścana - indeed, nothing can transcend that brahman. etadvai tat - this reality is indeed what naciketa has asked.