It is useful to examine the relation between syntactic production and logical implication
with one eye to what they have in common and another eye to how they differ.
The production says the appearance of the symbol
in a sentential form implies the possibility of replacing
with
Although that sounds like a possible implication, to the extent that
implies a possible
or that
possibly implies
the qualifiers possible and possibly are essential to the meaning of what is actually implied. In effect, those qualifications reverse the direction of implication, making
the best analogue for the sense of the production.
One way to understand a production of the form is to realize non‑terminal symbols have the significance of hypotheses. The terminal strings form the empirical matter of the language in question while the non‑terminal symbols mark the patterns or types of substrings which may be recognized in the linguistic corpus. If one observes a portion of a terminal string which fits the pattern of a sentential form
then it is an admissible hypothesis, according to the theory of the language afforded by the formal grammar, that the piece of string not only fits the type
but even comes to be generated under the auspices of the non‑terminal symbol
Resources
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