I'm reading some books on theory of knowledge and philosophy of mind. In those readings, notion of "open sentences" are used for certain extension, for example by Davidson.

Question: what does open sentence mean in philosophy?

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    Is there any chance I could persuade you to share a little more about your context? What books are you reading?
    – Joseph Weissman
    Commented Sep 14, 2015 at 14:54
  • @JosephWeissman Davidson's mental events. And D.W.Hamlyn's The theory of knowledge.
    – Darae-Uri
    Commented Sep 14, 2015 at 14:57
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    @Darae-Uri, more context would still be useful... can you edit your question with a passage for context? Also, in general, when someone asks for clarification, edit your question so that others who come across your question don't have to figure it out from the comment thread. Commented Sep 14, 2015 at 17:38
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    Yes, please provide one or two quotations involving "open sets", from both sources. Commented Sep 14, 2015 at 22:38
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    I found no "open sets" in Mental Events, there are plenty of "open sentences" but no "open sets" fitelson.org/proseminar/davidson.pdf
    – Conifold
    Commented Sep 15, 2015 at 2:42

1 Answer 1


An open sentence is a sentence featuring a free variable. They are sometimes called open formulas.

For example, 'x is green' is an open sentence, whereas 'Sam is green' is not.

Open sentences can be thought of as expressing a (possibly complex) property.

(I've checked and this is the sense being used in the Davidson paper mentioned.)

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