(In)equivalences between systems hold because of different properties of the systems themselves.
Logics = a formal way to express these properties.
- satisfiability relation states when a process satisfies a property
- enjoying the same properties coincides with being bisimilar

#### Example
![300](../../Pasted%20image%2020250505083500.png)
These processes are not bisimilar as:
- P1 can perform an action a followed by any b
- P2, after every a, can always perform an action b
- so there exists an a after which P1 cannot perform a b. But not for P2

### Syntax and Satisfiability