Sri Aurobindo translates Mundaka Upanishad, Chapter 3, Section 2, Verse 7: “The fifteen parts return into their foundations, and all the gods pass into their proper godheads, works and the Self of Knowledge, — become one in the Supreme and Imperishable.”
The manifested world consists of a number of parts (kalas) which take the Oneness of the Supreme and cause it to appear in different names, forms and actions, which eventuates in the awareness and actions of the individual beings.. The Prasna Upanishad discusses these parts at some length, although it mentions 16, and they are described as part of the larger enumeration of 24 found in the Sankhya. Some noted commentators consider the 15 parts to be the 5 breaths (Pranas), the 5 senses of perception and the 5 senses of action. Others substitute the 5 elements for the 5 Pranas. It is clear that through the passage of time, the exact enumeration intended by the Rishi has been lost or confused, while the general sense of the meaning remains.
The issue in this verse however is not to undertake a detailed examination of these parts, but to remind the seeker that everything arises from the Supreme and is a manifestation of the Supreme, regardless of the distinctions of name and form that we experience in the outer world through our various senses. Similarly the universal manifestation, as described in reference to gods and godheads, resolves itself into the Supreme as well. All action, all knowledge are manifestations of the Supreme, which pervades, permeates and constitutes all that occurs and is experienced in the world.
There is a state of awareness that consists of undifferentiated sense of Oneness, in which all names, forms, elements, sense impressions, objects of senses are resolved into their origin and no longer occupy the consciousness. In this state of awareness the overwhelming experience consists of that Oneness. Names, forms and circumstances either no longer register on the awareness or if they register, they are all seen as one ever-changing phantasmagoria superimposed upon this undifferentiated status, real only because of the reality of the Supreme which constitutes, creates and pervades these forms.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210