Combining Texts

You have chosen 'The Sign of Four' by Arthur Conan Doyle, 'Elements of Mathematical Logic' by Jan Lukasiewicz and 'New Essays on Human Understanding' by Gottfried Leibniz

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All the ideas for Modality, or select a subheading:
      A. Necessity
      B. Possibility
      C. Sources of Modality
      D. Knowledge of Modality
      E. Possible worlds