# Download An Essay In Modal Logic by Georg H. Von Wright PDF

By Georg H. Von Wright

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Extra resources for An Essay In Modal Logic

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E(F A & F A ) andE(PA&NF-A)andE(-FA&F-A) andE(NFA& F A ) . Of the four EV-constituents, however, the first expresses a proposition which is always false in virtue of the Principle of Non-Falsification. For, E ( P A & F - A ) means that there exists a thing - in the appropriate Universe of Discourse which is known to be neither A nor not-A, and this is an impossiN N N bility. Generally speaking, if there are n atomic predicates in a E V sentence, the sentence may have 2" V-constituents and 2(2n)E V constituents.

Mention is not being made of a person who possesses or does not possess knowledge. We could develop an alternative system in which the epistemic modalities are treated as “relative” to persons. I n this system “unknown to 9, we should have to deal with expressions like “known to d’, etc. Introducing quantifiers we should get a combined system dealing with expressions like ‘‘known to somebody”, “unknown to everybody”, etc. T h i s combination of epistemic and existential modalities will not be studied in the present essay.

Using the Principles of Permission, P-Distribution, and PExtensionality, the System P can be developed in close analogy to the previous systems for alethic and epistemic modalities. The truths of logic of the system are tautologies of the propositions expressed by certain P-constituents (P-tautologies). The decision problem can be solved using truth-tables and/or normal forms. We shall here mention some P-tautologies which concern the idea of commitment: 1. OA & O(A --f B) + OB. If doing what we ought to do commits us to do something else, then this new act is also something which we ought to do.