The previous two steps pointed out the purpose of blockchain in general and highlighted its importance for purely distributed peer-to-peer systems in particular. It turned out that maintaining integrity in distributed systems is the major purpose of blockchain. But why is maintaining integrity in distributed systems, and in purely distributed peer-to-peer systems in particular, such a challenge? This step answers that question by discovering the subtle relationship between trust and integrity of purely distributed peer-to-peer systems. As a result, this step will deepen our understanding of the importance of integrity and identify the major problem blockchain aims to solve. Finally, this step describes the environment in which blockchain is expected to provide the most value.