A more thorough explanation:

INDEX ANIMI SERMO

Speech is the index of the mind. This means that the way a person speaks can give insight into their thoughts and feelings. In legal terms, this maxim supports the idea that the language used in a law or contract is the best way to understand what the writer intended.

For example, if someone speaks in a very angry tone, it may indicate that they are upset or frustrated. Similarly, if a law uses very specific language, it can help lawyers and judges understand exactly what the lawmakers meant when they wrote the law.

Another example could be a contract between two parties. If the language used in the contract is unclear or ambiguous, it could lead to disagreements or legal disputes. However, if the language is clear and specific, it can help both parties understand their obligations and avoid any misunderstandings.

Overall, the maxim "index animi sermo" reminds us that the way we speak and write can reveal a lot about our thoughts and intentions, and that clear language is essential in legal documents.