The most work chain defines bitcoin

Bitcoin is defined as the chain with the most accumulated proof of work: whatever rules the most-work chain follows, that chain is Bitcoin, and a user who wants to stay on Bitcoin should follow it — installing new node software if necessary.

For (strongest version as argued): Proof of work is the system's only objective, external measure — everything else is opinion about rules. On this view, articulated plainly during the war, the most-work concept is what controls Bitcoin's governance: miners have real businesses and real invested capital, they produce the blocks, and hashpower is the economically weighted commitment of the industry's most exposed actors. The industry statement of the position was explicit — if a majority of miners adopt an upgrade, "by definition this is the new version of bitcoin," whatever chain has the most hashpower behind it. The strong form accepted its own implication: Bitcoin is the most-work chain whether your current node follows it or not, so remaining on Bitcoin may mean downloading new software that accepts the leading chain's rules. In every uncontested moment the definition works perfectly, since the most-work chain and the valid chain coincide — which, proponents argued, is exactly what a definition should do.

Against (strongest version as argued): Validity is checked before work is counted. The rebuttal recorded in the same source: nodes enforce rules, and a block that breaks them is ignored no matter who mined it — if miners created inflation above the 21 million limit and awarded themselves the coins, every node and wallet would treat that chain as invalid, and the coin following the original rules would continue as Bitcoin. Even light wallets enforce subsets of the rules. The reductio was contemporary, not retrospective: the idea that users would voluntarily install new software to follow a longer chain that stole coins from some users "made little sense," and the source's assessment is that large blockers pushed the most-work definition because it helped their cause, not because it described the system. The whitepaper itself, so often quoted for "vote with their CPU power," also says a majority attacker cannot make arbitrary changes or take money that never belonged to him, because "nodes are not going to accept an invalid transaction as payment." The corpus's verified mechanics agree: nodes-enforce-rules, miners-capabilities-limits.

Nuance: The defensible kernel is real: among chains valid under a node's rules, greatest cumulative work selects the active tip — that is consensus mechanics, not a claim. The contested move is promoting most-work from tiebreak to definition, from selecting among valid chains to defining validity itself. The source's most useful observation is that both readings are conditionally correct: had essentially everyone upgraded once a 75 percent threshold was hit, the miners would effectively have decided; if a significant minority of users refuse to upgrade, miners are not in control — and each camp assumed the rest of the world would behave like itself. The notes stop before the 2017 events usually cited as the empirical test; the corpus's segwit2x-cancellation entry flags (as analysis, not bare fact) the standard reading of how that test came out.

Sourcing: a single owner-research file — the owner's reading notes on The Blocksize War (Bier), a secondary account whose substantive coverage stops at February 2016 plus fragments — so INGEST_PLAYBOOK.md caps this at CONTESTED with confidence low. The source states both positions and its author's own lean; none of it was cross-checked against contemporaneous primary records this session.

Common misstatements: "Longest chain wins, so miners decide the rules" (work selects among valid chains; validity is checked first — see nodes-enforce-rules). "The whitepaper says CPU power votes on the rules" (the same paper says nodes reject invalid blocks and that majority hashpower cannot make arbitrary changes). "A fork that attracts majority hashrate becomes Bitcoin" (hashrate follows the chain the economy values, not the reverse — see miners-capabilities-limits; the 2017 episodes are the usual, contested, evidence). "A 51 percent attacker can rewrite the rules" (see fifty-one-percent-attack — reordering and double-spends of the attacker's own coins, not rule changes).