Letters

I felt surprised when Brutus decided to join the conspiracy when he received the letters that Cassius forged. Didn't he think that it could've been written by anyone? It didn't cross his mind that perhaps it was Cassius's scheme to lure him to join? In his first conversation with Cassius within the book, it is qutie evident that he didn't fully trust Cassius. Brutus should've felt suspicious. After all, why would someone send him a letter like that just soon after Cassius's offer to the conspiracy? I was surprised how quickly he made his decisions right after reading the letters Cassius forged.