Combining Texts

You have chosen 'New System and Explanation of New System' by Gottfried Leibniz, 'Marx' by Peter Singer and 'On the Foundations of Logic and Arithmetic' by David Hilbert

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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