David’s Uncle Axel catches him talking to himself when he is telecommunicating with Rosalind.
David is able to talk in his head with several others. It is a secret, because anything out of the ordinary is unacceptable in David’s society. When his Uncle Axel catches him, David realizes he is lucky. If it had been anyone else, he and Rosalind would have been expelled to the Fringes. Axel makes David promise to keep the communication a secret.
I want you to promise that you will never, never tell anyone else what you have just told me -- never. It's very important: later on you'll understand better how important it is. (ch 4)
This is the first time David begins to think about the importance of group identity. The telepaths do not even know each other’s names, except for David and Rosalind. They are all at risk. They need to communicate together in order to protect each other.