Bitcoin is a cryptocurrency system based on blockchain technology. Bitcoin’s transaction history is stored on the blockchain, a distributed ledger that contains all transaction information. In the Bitcoin system, the identification of past transactions is achieved by verifying transaction records on the blockchain.

Each node in the Bitcoin network will save a complete copy of the blockchain and verify and confirm transactions through a cryptographic algorithm. When a new transaction occurs, the transaction needs to be included in a block and connected to the previous block to form a chain by calculating hash values. Each block contains the hash value of the previous block, ensuring that each block is generated by the previous block and cannot be tampered with.

By verifying the hash value and digital signature of a transaction, the Bitcoin network ensures the authenticity and integrity of the transaction. If a transaction record is modified, its hash value will also change, causing other nodes on the blockchain to be unable to verify the validity of the transaction.

Therefore, the Bitcoin system identifies past transactions by verifying the transaction records in the blockchain. Only transactions that are verified and added to the blockchain are considered valid. This decentralized verification mechanism makes the Bitcoin system censorship-resistant and tamper-proof.