Consensus on blockchain: Establishing validity and truth (Non-technical)

How does blockchain consensus help to establish truth and validity? What is a consensus mechanism? What is forking a blockchain? What is the difference between “valid and true”? What are the limits to blockchain as a “truth machine”? What are the trade-offs between truth and the cost of verification? This video explores some of the issues relating to blockchain consensus and validity by using an analogy to a collectively owned message board. It includes discussion of the way that smart contracts might build in financial incentives for validation.