What has to be Understood?

The Blessed Buddha once insisted:
The nature of Ageing and Death has to
be Understood.
The Cause of Ageing and Death has to be Understood.
The End of Ageing and Death has to be Understood.
The Way to End
Ageing and Death has to be Understood.
The nature of Birth has to be Understood.
The Cause of Birth has to be Understood.
The End of Birth has to be Understood.
The Way to End Birth has to be Understood.
The nature of Clinging has to be Understood.
The Cause of Clinging has to be Understood.
The End of Clinging has to be Understood.
The Way to End Clinging has to be Understood.
The nature of Craving has to be Understood.
The Cause of Craving has to be Understood.
The End of Craving has to be Understood.
The Way to End Craving has to be Understood.
The nature of Feeling has to be Understood.
The Cause of Feeling has to be Understood.
The End of Feeling has to be Understood.
The Way to End Feeling has to be Understood.
The nature of Contact has to be Understood.
The Cause of Contact has to be Understood.
The End of Contact has to be Understood.
The Way to End Contact has to be Understood.
The nature of the 6 Senses has to be Understood.
The Cause of the 6 Senses has to be Understood.
The End of the 6 Senses has to be Understood.
The Way to End the 6 Senses has to be Understood.
The nature of Name-and-Form has to be Understood.
The Cause of Name-and-Form has to be Understood.
The End of Name-and-Form has to be Understood.
The Way to End Name-and-Form has to be Understood.
The nature of Consciousness has to be Understood.
The Cause of Consciousness has to be Understood.
The End of Consciousness has to be Understood.
The Way to End Consciousness has to be Understood.
The nature of Mental Construction has to be Understood.
The Cause of Mental Construction has to be Understood.
The End of Mental Construction has to be Understood.
The Way to End Mental Construction has to be Understood.
The nature of Ignorance has to be Understood.
The Cause of Ignorance has to be Understood.
The End of Ignorance has to be Understood.
The Way to End Ignorance has to be Understood.
These have to be Understood...


Source: The Grouped Sayings of the
Buddha. Samyutta Nikāya II 16
http://store.pariyatti.org/Connected-Discourses-of-the-Buddha_p_1379.html
https://What-Buddha-Said.net/library/ati_website/html/tipitaka/sn/index.html

Having cleaned and
polished the mind like a mirror one will understand!