Citing a secondary (indirect) quotation in the epigraph In my thesis I want to use the following quotation in the epigraph of a chapter:


  It is very difficult to predict — especially the future.


There are many variations of this quote, and the authorship is attributed to many different people. Very often it is assigned to Niels Bohr, and one of the oldest secondary sources I have found dates back to 1971.

How should I cite this secondary quotation in my epigraph?

Something like the following? —Niels Bohr (Mencher, 1971)
For something like this it is unlikely that you would be expected to cite the original source. Cite what you have found. 

An exception would be in research directly related to studies of sourcing and such, but I doubt that is your need here. 

Some things are lost in the mists of time, of course, and others are apocryphal.