Questions regarding eternity are considered to lack an answer:
According to the sutta quoted below, Buddha refrained from answering that type of questions. The reason for this comes from the buddhist notion of anatta, also mentioned below. The concepts are a bit hard to untangle, but this is the gist of it:
From the Pali suttas, the first that came to mind is "Assusutta (SN 15.3) -- Tears".
It's translated variously:
By Thanissaro Bhikkhu
From an inconstruable beginning comes transmigration.
By Ven. Sujato
Mendicants, transmigration has no known beginning.
By Piya Tan:
this cycle of life and rebirth is without a knowable beginning
The word in ...
For the benefit of all I would like to share my experience. Organized Concentration can be attained through practice. We can focus on the five aggregates or Truths and thus achieve concentration.
But natural concentration is also interesting. You find it everywhere. As child I craved for entertainment. I was highly concentrated while watching cartoon on TV. ...
I used to study Ajahn Buddhadasa very comprehensively. However, Handbook For Mankind was a book I was never partial towards. While it contains many basic teachings, I always found the verbal style of delivery difficult or abrasive to read. However, merely browsing and offering an answer:
The book is from 1956 lectures the Venerable Buddhadasa Bhikkhu gave ...
Concentration (ekaggatā), the mental focus on one object to the exclusion of all other objects is a universal mental state applicable to all mental states. I have concentration even when I type this answer.
It is most likely taken from the commentary to the satipatthana sutta, which has a similar list for each of the five hindrances. They are all worth reading: