Wealth can be lost. Reputation can be destroyed. Health fails. Power transfers. Every external good is held conditionally.
Letters 41, 71, 74, and 76 build a single argument from four directions: virtue — your capacity for self-governance, honest action, and rational judgment — is the only possession entirely within your jurisdiction and entirely immune to external removal.
Letter 41 establishes that the capacity for virtue is already inside you. It requires no acquisition, no credentials, no external validation. Letter 71 demonstrates that it functions as the highest good under any conditions — including conditions designed to destroy everything else. Letter 74 separates genuine goods from apparent goods. Letter 76 closes the argument: virtue is not merely sufficient for a good life. It is complete. Nothing needs to be added to it.