What are the thoughts that matter? They are the seemingly uncontrollable, idle thoughts ('If she wasn't married', 'I hate that guy...') that can annihilate what is good in us, what is godly, and do real damage in our world. Thoughts about food, sex, things, anger, dejection, acedia (weariness of soul), vainglory and pride. The fourth-century monk John Cassian described a method of training the mind not unlike that found in Eastern religions. In this book, a Benedictine nun and head of Monastic Interreligious Dialogue interprets this method in a contemporary way suitable for all serious about the spiritual life, devoting a chapter to each of the areas of thought that can prevent us achieving peace with God.