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9193
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ZF set theory has variables which range over sets, 'equals' and 'member', and extensionality [Dummett]
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Full Idea:
ZF set theory is a first-order axiomatization. Variables range over sets, there are no second-order variables, and primitive predicates are just 'equals' and 'member of'. The axiom of extensionality says sets with the same members are identical.
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From:
Michael Dummett (The Philosophy of Mathematics [1998], 7)
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A reaction:
If the eleven members of the cricket team are the same as the eleven members of the hockey team, is the cricket team the same as the hockey team? Our cricket team is better than our hockey team, so different predicates apply to them.
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9195
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Intuitionists reject excluded middle, not for a third value, but for possibility of proof [Dummett]
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Full Idea:
It must not be concluded from the rejection of excluded middle that intuitionistic logic operates with three values: true, false, and neither true nor false. It does not make use of true and false, but only with a construction being a proof.
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From:
Michael Dummett (The Philosophy of Mathematics [1998], 8.1)
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A reaction:
This just sounds like verificationism to me, with all its problems. It seems to make speculative statements meaningless, which can't be right. Realism has lots of propositions which are assumed to be true or false, but also unknowable.
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9186
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First-order logic concerns objects; second-order adds properties, kinds, relations and functions [Dummett]
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Full Idea:
First-order logic is distinguished by generalizations (quantification) only over objects: second-order logic admits generalizations or quantification over properties or kinds of objects, and over relations between them, and functions defined over them.
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From:
Michael Dummett (The Philosophy of Mathematics [1998], 3.1)
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A reaction:
Second-order logic was introduced by Frege, but is (interestingly) rejected by Quine, because of the ontological commitments involved. I remain unconvinced that quantification entails ontological commitment, so I'm happy.
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9187
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Logical truths and inference are characterized either syntactically or semantically [Dummett]
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Full Idea:
There are two ways of characterizing logical truths and correct inference. Proof-theoretic or syntactic characterizations, if the formalization admits of proof or derivation; and model-theoretic or semantic versions, being true in all interpretations.
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From:
Michael Dummett (The Philosophy of Mathematics [1998], 3.1)
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A reaction:
Dummett calls this distinction 'fundamental'. The second one involves truth, and hence meaning, where the first one just responds to rules. ..But how can you have a notion of correctly following a rule, without a notion of truth?
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12741
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If experience is just a dream, it is still real enough if critical reason is never deceived [Leibniz]
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Full Idea:
Even if this whole life were said to be only a dream, and the visible world only a phantasm, I should call this dream or phantasm real enough if we were never deceived by it when we make good use of reason.
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From:
Gottfried Leibniz (De modo distinguendi phaenomena [1685], A6.4.1502), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 7
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A reaction:
I find this response more satisfactory than his response in Idea 12740. As a supporter of the coherence account of justification, I take the closest we get to knowledge to be when our full critical faculties and experience are brought to bear, and shared.
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12740
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The strongest criterion that phenomena show reality is success in prediction [Leibniz]
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Full Idea:
The most powerful criterion of the reality of phenomena, sufficient even by itself, is success in predicting future phenomena from past and present ones.
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From:
Gottfried Leibniz (De modo distinguendi phaenomena [1685], A6.4.1502), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 7
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A reaction:
I would say that this is clutching at straws, as there is no reason at all to deny that dreams could be thoroughly coherent and predictable in their events. We must just live with these doubts, not try to defeat them.
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12721
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Light, heat and colour are apparent qualities, and so are motion, figure and extension [Leibniz]
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Full Idea:
Concerning bodies I can demonstrate that not merely light, heat, color, and similar qualities are apparent but also motion, figure, and extension.
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From:
Gottfried Leibniz (De modo distinguendi phaenomena [1685], A6.4.1504), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 4
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A reaction:
Leibniz is not consistent on this. Here he is flirting with idealism, but he often backs away from that. In Discourse §12 he makes secondary qualities certainly subjective, and primary qualities possibly so. He admits the primaries contain eternal truths.
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