The relative strength of two systems of formal logic can be defined via model theory. Specifically, a logic ${\displaystyle \alpha }$ is said to be as strong as a logic ${\displaystyle \beta }$ if every elementary class in ${\displaystyle \beta }$ is an elementary class in ${\displaystyle \alpha }$. ^[1]

See also

• Abstract logic
• Lindström's theorem

References

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