Combining Texts

You have chosen 'Time Without Change' by Sydney Shoemaker, 'An Axiomatization of Set Theory' by John von Neumann and 'Letters to Burcher De Volder' 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