The previous step explained the relationship between purely distributed peer-to-peer systems and the most prominent use case of blockchain as a means to manage ownership. As a result, you learned that the integrity of a distributed peer-to-peer system of ledgers is its ability to make true statements about ownership and to ensure that only the lawful owner can transfer its property rights to others. But what does this statement mean in real life? What happens if integrity is violated? This step considers these questions in more detail. In particular, this step introduces one of the most important examples of violated integrity in distributed peer-to-peer systems: the double-spending problem.