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It indeed generalizes the Axiom of propositional extensionality mentionned by Church 1940, which states that two logically equivalent propositions are equal.
Some understand this as the warrant failing as a warrant for equivalent propositions.
Russell, however, sticks to an intensional point of view, stressing that equivalent propositions often can be quite different.
Jaynes' invariance condition bids us to assign equal probabilities to equivalent propositions, reformulations of one another that are arrived at by such admissible transformations of our problem.
Interestingly this is connected there to the question of the identity of equivalent propositions and of the logical product of a class of propositions.
But as a first approximation to the concept, we will assume that it is not hyperintensional and that logically equivalent propositions have the same degree of truthlikess.
Similar(47)
Conversion yields an equivalent proposition (and is hence a valid inference) in general only with so-called E and I propositions (universal negatives and particular affirmatives).
It seems that the equivalent proposition is a known fact.
They include assertibles (the Stoic equivalent for propositions), imperativals, interrogatives, inquiries, exclamatives, hypotheses or suppositions, stipulations, oaths, curses and more.
Vendler proposed that a fact is 'an abstract entity which indiscriminately contains a set of referentially equivalent true propositions' (Vendler 1967, 711; my emphasis).
For example, proposition #15 there is not equivalent to proposition #22, despite superficial syntactic similarity.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com