A
formal system is broadly defined as any well-defined
system of abstract thought based on the model of
mathematics. The
entailment of the system by its logical foundation is what distinguishes a formal system from others which may have some basis in an abstract model. Often the formal system will be the basis for or even identified with a larger theory or field (e.g.
Euclidean geometry) consistent with the usage in modern mathematics such as
model theory. A formal system need not be mathematical as such; for example, Spinoza's
Ethics imitates the form of Euclid's
Elements.