In public ledger blockchains, immutability is a key design property which provides [[Censorship resistance|censorship resistance]] or [[Finality|finality]]. This property is cemented by the implementation of a [[Consensus Mechanism|consensus mechanism]], namely its cryptographic implementation and game theory incentives. As a result, blocks of transactions become effectively irreversible as time passes. Such an irreversible state can largely be attributed to a core principle of the Bitcoin protocol, namely [[Decentralization|decentralization]]. >[!idea] Decentralization both deters and disincentivizes collusion, allowing blockchains to remain immutable, incorruptible, and censorship resistant. The property of immutability is extended to any data that is deployed on chain, whether it be transactions or [[Smart Contracts|smart contracts]]. On [[Proof of Work]] blockchains, what truly locks in immutability lies in the [[Chain Concepts/Tokenomics/Thermodynamic Cost|thermodynamic cost]] of a reversal. The cost of competing blocks (i.e., a malicious chain) is generally greater than the benefit of continuing work honestly. Thus, network integrity is more than an incentive for its participants; it is the key its own survival. Immutability then relies heavily upon finality, meaning that blocks cannot be (easily) altered or reversed. >[!examples] > >Finality in the Bitcoin protocol is considered [[Finality#Probabilistic Finality|probabilistic finality]]; it relies upon the probability that no blocks will be reversed by a malicious or bad actor. The more time elapsed since a block is added to a chain, the greater the probability that block will never be reversed. In this way, Bitcoin is hardened against a [[51% Attack]] and game theory problems like the [[Byzantine Generals Problem]]. > >[[Proof of Stake]] blockchains often use a different method for finality which can be referred to as [[Finality#Deterministic Finality|deterministic finality]], meaning that all blocks are final irrespective of any attack (see [[Finality#Single-slot Finality|Single-slot finality]]). These implementations will typically be found in more centralized protocols, often deploying consensus algorithms which use [[Practical Byzantine Fault Tolerance]]. Because [[Mining|mining]] and [[Staking|staking]] rewards and penalties (dis)incentivize collusion attacks - making network attacks very costly - a system which meets these requirements is considered to be anti-fragile. In this context, [[Anti-fragilit|anti-fragility]] means that a system failure - caused by a network attack or some random system malfunction - can theoretically be recovered or rebounded from. >[!tip] An anti-fragile network can emerge stronger and more resilient as a result of an attack/failure. In this way, consensus protocols make blockchains fault tolerant systems by aligning profit motives with security models, such that nodes prefer to spend resources competing rather than colluding.