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16 votes
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Why the reflection rule trivializes higher paths in Martin-Löf Extensional Type theory?

The point is that the reflection rule makes $p = \mathsf{refl}_x$ a well-formed expression. This turns out to be incredibly dangerous: now we can prove it by induction on equality. More precisely: …
Zhen Lin's user avatar
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10 votes
Accepted

How should I be thinking about object classifiers / universal fibrations / universes?

Yes, the universe is the classifying space for small homotopy types. For various reasons, the universe is not itself a small homotopy type; so it fails to be an object classifier for the trivial reaso …
Zhen Lin's user avatar
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8 votes
1 answer
638 views

The independence of path induction

In §1.12 of the Homotopy type theory book, it is mentioned that indiscernibility of identicals is a consequence of path induction. More precisely, for each type $C$ dependent over a type $A$, there is …
Zhen Lin's user avatar
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7 votes
1 answer
204 views

Finitistic interpretation of Nelson's internal set theory

What does “standard” in internal set theory really mean? Is it secretly a way of reconciling conventional mathematics with (ultra)finitism? Until recently I thought “standard” was just a way of talkin …
Zhen Lin's user avatar
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