W. Somerset Maugham, The happy man stories

"I have always hesitated to give advice, for how can one advise another how to act unless one knows that other as well as one knows himself? Heaven knows. I know little enough of myself: I know nothing of others. We can only guess at the thoughts and emotions of our neighbours. Each one of us is a prisoner in a solitary tower and he communicates with the other prisoners, who form mankind, by conventional signs that have not quite the same meaning for them as for himself."