To question a quotation...

For some time now, I have admired the quotation:

For those who believe, no explanation is necessary;
For those who do not believe, no explanation is possible.

Having recently noticed that many a magician and mentalist were attributing this quotation to Joseph Dunninger, the 'father of mentalism" I had taken it for granted.

Today in trying to find out if this attribution indeed were true, I came across many websites that were attributing this quote to numerous others. While Quotation-related websites either did not list this quote, or attributed it to "Author Unknown", like in the case of QuoteDB.

I for one would really like to know the original author of this beautiful quote. Anybody in a position to help?

Nakul