Combining Texts

You have chosen 'Counterpart theory and Quant. Modal Logic' by David Lewis, 'The Foundations of Mathematics' by David Hilbert and 'Exigency to Exist in Essences' 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