What does Liu Bowen's thirteen-character calligraphy mean?

Liu Bowen wrote thirteen letters to Zhu Yuanzhang, who was seriously ill. Thirteen letters from Liu Bowen were written by Liu Bowen, a famous general, politician and writer in Ming Dynasty in China, to Zhu Yuanzhang, who was seriously ill. These letters are profound in content and rich in meanings, which not only express Liu Bowen's loyalty and anxiety to the Ming Dynasty and Zhu Yuanzhang, but also reflect his profound thinking on the national politics and social reality. Among them, calligraphy is exquisite and has become a work of art.