The Skeleton and Body of Bitcoin: Deep Support for PH Computational Complexity Theory In the vast landscape of blockchain technology, Bitcoin is undoubtedly an insurmountable monument. For many people, blockchain seems to be the entirety of Bitcoin, but delving deeper into its core, we can see a more intricate structure: blockchain technology is just the "body" of Bitcoin, while the "PH (Polynomial Hierarchy) three-layer structure" in computational complexity theory is the "skeleton" that supports this magnificent building of Bitcoin. PH three-layer structure: the theoretical cornerstone of Bitcoin To understand this, we first need to touch upon a core concept in computer science - computational complexity theory. This theory classifies computational problems based on the amount of resources (time or space) required to solve them. **PH (Polynomial Hierarchy) * * is one of the important hierarchies that describes the categories of computational problems based on polynomial time Turing machines and alternating quantifiers. The strength and uniqueness of Bitcoin lies precisely in its clever design that embeds this PH three-layer structure: Level 1: UTXO (Unspoken Transaction Output) - Verifiable Output This corresponds to the * * "NP" * * (Non Deterministic Polynomial Time) problem in the PH level. A UTXO represents a disposable, unused Bitcoin in the digital world. Its existence is verifiable: anyone can easily verify whether a UTXO truly exists and has not yet been spent. But to 'create' a new UTXO (i.e. conduct a valid transaction), a computational problem needs to be solved, which is the starting point of Bitcoin trading. The second layer: PoW (Proof of Work) - the provable workload, which corresponds to the * * "coNP" * * problem in the PH level, or more precisely, solving an "NP hard" problem. The proof of work mechanism in the mining process requires miners to invest huge computing resources to try to find a random number (nonce) that meets specific conditions. Verifying whether this' workload 'has been completed (i.e. whether the block is valid) is extremely easy and fast. But finding this random number that meets the criteria is an extremely difficult and unpredictable process, which is the essence of PoW. This process ensures the security of the network and prevents malicious attacks. Third layer: Longest chain - tamper proof consensus. This corresponds to higher levels in the PH hierarchy, as it involves selecting the optimal path under uncertainty. The Bitcoin network achieves consensus and resolves forks through the 'longest chain principle'. This means that miners will always continue mining on the chain with the highest known workload. The length of this chain represents the accumulated computational workload and is therefore considered the 'correct' and safest chain. Attempting to tamper with historical transactions means recalculating and surpassing all PoW workloads on the current longest chain, which is almost impossible computationally, thus ensuring the finality and immutability of transactions. It is these three closely connected and progressively complex computational structures that together form the unbreakable "skeleton" of Bitcoin. Blockchain: The 'Body' that Fills the Skeleton When we talk about blockchain, we usually refer to its organizational structure of transactions (Tx)/blocks/chains. This structure itself provides a distributed ledger approach for packaging, connecting, and forming time series of transaction data. However, this structure is like an empty shell that needs to be filled with specific mechanisms to function. It is the PH three-layer skeleton of Bitcoin that gives these structures life and meaning: The UTXO system determines the final state and validity of transactions, filling the core logic of * * transactions (Tx) * *. The PoW mechanism determines the generation method and verification process of blocks, filling in the casting and security of blocks. The principle of longest chain determines the extension of the chain and the achievement of consensus, filling in the integrity and irreversibility of the chain. Therefore, the Tx/Block/Chain organizational structure referred to in blockchain is precisely filled and strengthened by the UTXO/PoW/longest chain in the PH three-layer skeleton. The flaw of altcoins: only physical body, no skeleton This is the fundamental reason why Bitcoin is "awesome" and also the essential difference between it and most "altcoins". Many cryptocurrency projects, although also adopting blockchain technology and possessing the "physical" structure of Tx/Block/Chain, often lack or fail to deeply understand and effectively apply the "skeleton" of PH computational complexity theory. These altcoins may have innovative consensus mechanisms (such as PoS, DPoS, etc.) and enhanced programmability (such as smart contracts), but they often cannot be compared to Bitcoin's PH three-layer framework in terms of underlying security, decentralization, and ability to resist large-scale attacks. They may achieve the functionality of a 'distributed ledger', but they may not provide the same level of 'finality without intermediaries' and' immutability '. In other words, they only have the "physical body" of blockchain - visible structure and application level, but lack the mathematical and logical framework endowed by computational complexity theory, which has been tested over time and is difficult to break through. This makes them more susceptible to the influence of centralized entities or potential vulnerabilities in security when facing real challenges. The uniqueness of Bitcoin lies not only in its technological invention, but also in its masterpiece of perfect integration of engineering and cryptography, with its underlying logic deeply rooted in the solid foundation of computational complexity theory. This makes it not just a 'digital currency', but also a profound social experiment, a truly 'free infrastructure without intermediaries'.