Step 1: The context.
In the story The Third Level, Charley was always searching for evidence that the third level of the Grand Central Station actually existed. His search for truth and validation was continuous.
Step 2: Discovery of the letter.
One day, while sorting out his old collection of first-day covers, Charley unexpectedly found an envelope. It was hidden among his grandfather’s stamp collection. The envelope carried a letter dated July 18, 1894.
Step 3: The contents of the letter.
The letter was written by his psychiatrist friend, Sam. It mentioned that Sam had reached the third level and found it to be real. He described enjoying life in Galesburg, a peaceful town of the past, and advised Charley and his wife Louisa to keep looking for the third level as it was a wonderful escape from the harsh realities of the modern world.
Conclusion:
Thus, Charley found Sam’s letter while looking through his old stamp collection, and the letter confirmed the existence of the third level, blending fantasy with reality.