I have read this story a long time ago. I remember some fragments like this:
There is a young man travelling (maybe on a train). He starts talking to another passenger in the same seat (or cabin). The other passenger has an apple. The young man wants the apple but the stranger warns him that this is an apple of wisdom. After eating it he will know about everyone's feelings and intentions. The young man insists and the stranger gives him the apple before the young man gets off the train. After eating the apple, the young man is shocked. He suddenly starts "seeing" how other people like his lover/wife and other relations are not good to him.
For a long time (around 1990), I thought that this is an "Oscar Wilde" story. But after all the time I am not able to find it in any Oscar Wilde collection (the ones I came across). Also, it was in English.
If you can help me figure this out, I would like to read it again.