Definition:Substitution Instance
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Definition
A substitution instance of a sequent $S_1$ is a sequent $S_2$ which results from $S_1$ by replacing one or more of the statement variables in some statement forms in $S_1$ throughout the whole of $S_1$ by other statement forms, as follows:
- Every occurrence of a particular substituted statement variable is replaced by the same statement form;
- Different statement variables may be replaced by different statement forms;
- The substitution can be made only on statement variables.
Sources
- Donald Kalish and Richard Montague: Logic: Techniques of Formal Reasoning (1964): $\text{II}: \S 4$
