Final answer:
Gerda deduces Arthur's changing state of mind from his letters by analyzing the tone, content, and language used in them.
Step-by-step explanation:
Gerda deduces Arthur's changing state of mind from his letters through analyzing the tone, content, and language used in his correspondences. For example, if Arthur's letters were previously filled with happiness and excitement but now contain sadness or despair, Gerda can infer a shift in his mental state.
Additionally, if Arthur's writing style becomes disjointed or erratic, it may indicate a decline in his mental well-being. By closely examining the details and nuances of the letters, Gerda can gain insight into Arthur's changing state of mind.