This quote is attributed to Publilius Syrus, a Roman writer of mimes, and not Seneca. It's often misattributed, as the sentiment is similar to other Seneca quotes.
The meaning of this quote is that the way a person speaks reveals their character and intelligence. It emphasizes the importance of careful and thoughtful communication.
Here are some of Seneca's similar quotes that deal with speech:
* "A man's character is shown in his speech."
* "The tongue is a dangerous weapon."
* "Words can wound more deeply than any sword."
So while Seneca didn't say "Speech is the index of the mind," he did have similar thoughts on the power and importance of language.