Blockchains: neither public, nor private.
Public associations of volunteer servers as consensus hosting providers.
A Bitcoin node is a volunteer client. Wikipedia is a volunteer server.
A volunteer client is a “Peer-to-Peer” entity: a socially anonymous entity. You know the node by a network address (often ephemeral).
A volunteer server is an entity you socially know. We can know who the people behind Wikipedia are, as they publish who they are in the website itself, reachable by a DNS lookup for “www.wikipedia.com.”
Volunteer, “Peer-to-Peer” clients don’t trust each other and don’t know each other. So-called “Peer-to-Peer” networks are masquerade balls. Anonymous machines cooperating directly with each other.
“Blockchain” networks such as Bitcoin and Ethereum are Peer-to-Peer networks run by volunteer “clients” — even if the “core” nodes of the network are run as “servers,” with fixed IP addresses and high availability. Joining and leaving the Bitcoin network is entirely free. It is a completely public blockchain.
Banks and other types of corporations are looking into “private” blockchains: protocols where the “peers” are all known private participants. Private parties reach a consensus that serves some business purpose of these private participants. No one else can join or benefit directly from the data.
Public blockchains are slow and redundant: because no single replicator of the public ledger can be trusted, the network can only trust a consensus of all of them at once. The level of trust is so low that nodes have to fight each other into a “mining” arms-race and war to converge into an event ordering mechanism.
There’s an easier and more efficient thing that can be done to serve the public interest. We can remain open to users, we can efficiently and securely scale consensus serving to all who need it, openly. We can reap the benefits of “private” blockchains without its main penalty, that is, that it is a closed system with respect to benefiting from consensus, from being able to deploy consensus programs (“contracts”) and from being able to use them.
The change is that each instance of a blockchain, each network, must not be made of anonymous peer-to-peer clients, but of associations of volunteer, known servers.
Think several known, “free,” Wikipedia-like websites cooperating to serve you, instead of several private banks or other for-profit corporations keeping you out, and instead of a masquerade ball of anonymous nodes.
Wikipedia is a public service operated by a private agreement. And you’re going to see a consensus engine, a “blockchain” that is like a collection of Wikipedias: a public, social agreement between multiple server operators to run a “private” blockchain where only the servers can sign “blocks” (consensus, state updates) into it… but that accepts everyone’s transactions and “contracts,” just like Bitcoin, Ethereum and all other public and anonymous blockchains do.
Instead of a masquerade ball, an efficient and secure blockchain can be run by a group of non-anonymous citizens who volunteer for the greater good.
These multi-server networks can run protocols that are several orders of magnitude simpler, faster and more efficient than what a “public blockchain” system can achieve.