Combining Texts

You have chosen 'Investigations in the Foundations of Set Theory I' by Ernst Zermelo, 'Maxims and Reflections' by Wolfgang von Goethe and 'An Introduction to Modal Logic' by GE Hughes/M Cresswell

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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