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| \end{matrix}</math> | | \end{matrix}</math> |
| |} | | |} |
| + | |
| + | It is useful to examine the relationship between the grammatical covering or production relation <math>(:>\!)</math> and the logical relation of implication <math>(\Rightarrow),</math> with one eye to what they have in common and one eye to how they differ. The production <math>q :> W\!</math> says that the appearance of the symbol <math>q\!</math> in a sentential form implies the possibility of exchanging it for <math>W.\!</math> Although this sounds like a ''possible implication'', to the extent that ''<math>q\!</math> implies a possible <math>W\!</math>'' or that ''<math>q\!</math> possibly implies <math>W,\!</math>'' the qualifiers ''possible'' and ''possibly'' are the critical elements in these statements, and they are crucial to the meaning of what is actually being implied. In effect, these qualifications reverse the direction of implication, yielding <math>^{\backprime\backprime} \, q \Leftarrow W \, ^{\prime\prime}</math> as the best analogue for the sense of the production. |
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| <pre> | | <pre> |
− | It is useful to examine the relationship between the grammatical covering
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− | or production relation ":>" and the logical relation of implication "=>",
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− | with one eye to what they have in common and one eye to how they differ.
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− | The production "q :> W" says that the appearance of the symbol "q" in
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− | a sentential form implies the possibility of exchanging it for "W".
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− | Although this sounds like a "possible implication", to the extent
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− | that "q implies a possible W" or that "q possibly implies W", the
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− | qualifiers "possible" and "possibly" are the critical elements in
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− | these statements, and they are crucial to the meaning of what is
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− | actually being implied. In effect, these qualifications reverse
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− | the direction of implication, yielding "q <= W" as the best
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− | analogue for the sense of the production.
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− |
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| One way to sum this up is to say that non-terminal symbols have the | | One way to sum this up is to say that non-terminal symbols have the |
| significance of hypotheses. The terminal strings form the empirical | | significance of hypotheses. The terminal strings form the empirical |