Consensus determines truth in blockchain because the only valid ledger on a blockchain is the one where the majority of nodes on the network share and broadcast the same information, therefore reaching consensus.
In a central database, truth is determined by those who control it. If there is only one party in control of a central database, they can change the inputs of a database to reflect whatever they consider true. Truth is most likely derived from a central source with the ability and incentive to alter it. In blockchain, truth is derived from numerous distributed sources that, for the most part, cannot be altered and have little incentive to do so.
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