It has been said that Lord Brahma taught munDakOpanishat to his son Adharva who told it to Angiru. Satyavahu received the teaching from Angiru who in turn passed it on to Angirasu (of Bhaaradhwaaja). So far the upanishat was transmitted to bachelors and sanyaasis. Eventually it reached a gruhastu called Mahasyaala. Therefore there is no reason to believe that munDakOpanishat is meant only for sanyaasis.
Mundanam means to block all thoughts and focus on the teaching. Mahasyaala wanted to know what is it that when known will make all the knowledge known? Behind this question, there is a well known concept called aparaavidya which encompasses vEdas, various professional skill-based knowledge like sculpting, carpentering, etc. Thus the aparaavidya includes even vEdas with their upaasanaas and karma kanda that are considered inferior to paraa vidya. So what is vidya (vid means to know)? It is that which brings the akshara (unlimited) tattva to experience (anubhava). Whereas aparaavidya includes theory and practice, paraavidya is only theory that brings knowledge to anubhava. One can perform a yajna and gain the knowledge or read about it and attain anubhava.
Before we go in to the depth of the upanishat, we need to know about akaLam and sakaLam. SakaLam means made of parts and akaLam is the opposite. A tree has several parts like branches, leaves, fruit, flowers, etc. It is a divisible whole. It is also called swagata bEdha.
Then there are sajaateeya and vijaateeya. Sajaateeya means of the same species. A person and his wife are sajaateeya. Vijaateeya means of different species. Whereas a person and a cow are vijaateeya.
AkaLam means there are no parts, is shapeless and complete in itself. Aatma is akaLam or nishkaLam. One has to gain the knowledge about aatma by making his manas akalam because it then will be sajaateeya with aatma. Only advaitin can comprehend the omnipresent aatma that is nishkaLam.
The prasna upanishat coined a word called “pari drashtu” which means to see from all sides. One has to see the sthaana (space) rather than the sthaani (object) that is immersed in space.
By analogy, there is “pari darsana” which means to experience or bring to anubhava. One can perform sravana-manana-nidhidhyaasana (listen-ruminate-meditate) over gnaana. But it is not complete. One has to also attain darsana. By that it is meant that one has to look at the sthaana surrounding the sthaani.
The budhi or intelligence dawns on one at a 4 or 5 years of age normally. Until then the child has no knowledge of sthaana. It only sees the objects. Similarly an old person who loses his budhi doesn't see the sthaana. The sthaana can only be seen in the middle life with saadhana as one is accustomed to seeing only sthaani or objects. In paridarsana one trains oneself to not look at naama-roopa-kriya (name-form-action) but instead the sthaana or the space occupied by them.
The upanishat also suggests one should approach a guru to receive the darsana as aagama or that which is traditionally passed down or a sampradaaya. The knowledge of aatma is better gained in a guru kula but not a must.
The underlying knowledge/gnaana is like sakti that remains hidden in a person. When it is made extroverital it is called anushtaanam. The upanishad talks about “pari darsana” which means in this context to block all the thoughts about aparaavidya and focus on paraavidya alone.
A bhoota yOni has been posited that is considered as the source for the origin of all objects such as mountains, rivers, life forms, etc. When one believes in the bhoota yOni it is considered as aparaavidya. Because there is no such a thing as per paraavidya.
As per paraavidya, the universe was neither created nor recreated after pralaya. Those who believe in the creation of the universe belong to the poorva meemaamsa camp of the vEdaas. They perform karma and upaasana. According to the paramaartha drushti there is no praana or manas. Everything is known to one by jnaana. For example, the observation that sky is black is untenable. The sky adorns all colors depending on how the atmosphere alters our perception through optics.
Is the world real? We are called jeeva who transact with the world or jagat. In other words, a pindaandam (i.e. self) deals with brahmaandam (world). So what is self? It is gnaana that which interacts with the world. The world is made of naama-roopa-kriya or name-form-action. The thoughts arising in self are called kaLas (parts). The kaLas are there in all the jaagrat-swapna-sushupti avasthaa-trayam (wakeful-dream-deep-sleep). The jeeva in the natural state is sakaLam. The world is also sakaLam. Thus a sakaLam deals with another sakaLam. But the upanishat says there is akaLam (devoid of kaLa or parts) which is aatma that is made of nothing but gnaana.
The upanishat exhorts that one has to turn the mind away from sakaLam and focus on nishkaLam to realize that there is only Brahman. There is no viswa but vibhu.
So how does one attain this state of being? By rejecting karma and upaasanaas ordained by the vEdas. One need only perform those karmas that are necessary to lead a normal life. For example, washing, eating, drinking, etc. There is no need to perform the karmas described in the vEdaas. This includes worshiping gods and performing rituals to please them.
The upanishat then talks about adhishtaanam which is one of the pancha bhootas known as sky or space. The rest of the bhootas (earth, water, air and fire) were superimposed on the sky/space which has independent existence and bears the four bhootas.
Finally, we note that when an image is seen in a mirror (pratibimba), it is an illusion. This is called aabhaasa which can be destroyed. The bimba or the object projected on the mirror remains as ever. The manas is like the mirror that has been covered by dirt which is aparaavidya. The take home message from the upanishat is: one has to attain paraavidya and cleanse the thoughts that were made impure by aparaavidya.
