1. One of the four logical ways in which perception, memory, or representation can perform a significant transformation of its source material. Substitution involves replacing some feature from an original stimulus with another which was not. An example of this being done very consciously is the commutation test. See also addition; deletion; transposition.
2. (rhetoric) Immutatio, one of Quintilian's four types of rhetorical figures of speech involving deviation (mutatio): in this case, the substitution of elements, e.g. metaphor, metonymy.