What is 'circular security' in Proof-of-Stake?

Circular security is a term for the situation where a Proof-of-Stake network is secured by the value of its own staked tokens — tokens whose value is assigned by that same network. In Proof-of-Stake, the cost of attacking the network is internal to the system: to attack, you must acquire staked tokens, but those tokens only have value because the network says they do. By contrast, Proof-of-Work security costs are external, existing in the physical world as electricity and hardware regardless of what value the network assigns to itself. Understanding this distinction matters because PoW and PoS differ fundamentally in where the cost of attack comes from — not merely in how much energy they use.