Einstein and Michele Besso

Einstein never said "Reality is merely an illusion, albeit a very persistent one." The reality behind this quote can be traced to Michele Besso, a Swiss/Italian engineer and close friend of Einstein during his years at the Swiss Federal Polytechnic in Zurich, and later at the patent office in Bern. Besso introduced Einstein to the works of physicist and philosopher Ernst Mach, who influenced Einstein greatly. 

Einstein ends his paper on special relativity by saying that he is indebted to Besso for several valuable suggestions. Einstein remarked "I could not have found a better sounding-board in the whole of Europe." In this way Einstein and Besso became inseparable.

Upon Besso's death in 1955, Einstein wrote a letter of condolence to the Besso family—less than a month before his own death—which contained the following quote "Now he has departed from this strange world a little ahead of me. That signifies nothing. For those of us who believe in physics, the distinction between past, present and future is only a stubbornly persistent illusion."