Ideas from 'Modal Logic within Counterfactual Logic' by Timothy Williamson [2010], by Theme Structure

[found in 'Modality' (ed/tr Hale,B/Hoffman,A) [OUP 2010,]].

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4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / h. System S5
In S5 matters of possibility and necessity are non-contingent
10. Modality / A. Necessity / 1. Types of Modality
Necessity is counterfactually implied by its negation; possibility does not counterfactually imply its negation
10. Modality / B. Possibility / 8. Conditionals / a. Conditionals
Strict conditionals imply counterfactual conditionals: □(A⊃B)⊃(A□→B)
10. Modality / B. Possibility / 9. Counterfactuals
Counterfactual conditionals transmit possibility: (A□→B)⊃(◊A⊃◊B)
10. Modality / C. Sources of Modality / 1. Sources of Necessity
Rather than define counterfactuals using necessity, maybe necessity is a special case of counterfactuals
15. Nature of Minds / C. Capacities of Minds / 2. Imagination
Imagination is important, in evaluating possibility and necessity, via counterfactuals