Step 1: Understanding the Concept:
This question refers to the advice given by a mother to her daughter in the story 'Two Horizons', which is told through their correspondence.
Step 2: Detailed Explanation:
The daughter's letters are filled with complaints about her hard life and lack of joy. The mother, from her more experienced and peaceful perspective, tries to guide her.
A central piece of the mother's wisdom is that material wealth is not the key to a good life. She advises her daughter that true, genuine happiness is a state of mind and spirit. It is something that comes from within and cannot be purchased with money or possessions.
Step 3: Final Answer:
The mother in her letter tells the daughter that happiness cannot be bought.