Logic is defined in various ways. For instance, Petrus Hispanus a medieval logician said that dialectica (that is, logic) est art artium et scientia scientiarum ad omnium aliarum scientiarum methodorum principia viam habent (logic is the art of all arts and science of sciences, which provides methods for all other sciences). This definition indirectly implies that logic is fundamental and universal. The paper concerns logic in its formal or mathematical understanding. Logic and its properties are analyzed via metalogical devices, particularly – the concept of consequence operation. More specifically, logic (as a logical system) is defined as the set of consequences of the empty set. This approach corresponds to the idea that theorems of logic are universally true (true in all models or possible worlds).
