"Time present and time past
Are both perhaps present in time future,
And time future contained in time past.
If all time is eternally present
All time is unredeemable.
What might have been is an abstraction
Remaining a perpetual possibility
Only in a world of speculation.
What might have been and what has been
Point to one end, which is always present.
Footfalls echo in the memory
Down the passage which we did not take
Towards the door we never opened
Into the rose-garden. My words echo
Thus, in your mind."

Extract from Burnt Norton by TS Eliot (part of the Four Quartets series)

See also the fascinating life and work of the somewhat eccentric John William Dunne, an aeronautical engineer by trade, and speculator on non-linear time by hobby, born in Co. Kildare in 1875.

(Image courtesy of: http://acepilots.com/airplanes/country/british/dunne-biplane/)