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| 8859 | The main modal logics disagree over three key formulae [Yablo] |
| Full Idea: Lewis's different systems of modal logic differed about such formulae as □P implies □□P; ◊□P implies □P; and ◊S implies □◊S | |
| From: Stephen Yablo (Apriority and Existence [2000], §06) | |
| A reaction: Yablo's point is that the various version don't seem to make much difference to our practices in logic, mathematics and science. The problem, says Yablo, is deciding exactly what you mean by 'necessarily' and 'possibly'. |