Hey all,

   As I'm looking into metamath there is something lingering that is making 
me uncomfortable: definitions are axioms.

   This is obviously intended and by design by the community (the book has 
a whole chapter about it), but I'm still uncomfortable with the 
justifications and consequences.

    A few irrational feelings I think I'm having:

    - I can't parse or understand df-bi trivially. What does it mean?
    - How do I know that the axioms of the definitions aren't introducing 
contradictions?
    - It seems that definitions aren't supposed to "extend the language", 
but I don't get that sentiment as I read them
    - We say "all you need is propositional and first order logic and set 
theory axioms" but then proceed to introduce a bunch by definitions

    I'm sure this can't be a new sentiment, since an entire chapter in the 
book was dedicated to it, but I was wondering if:

    (a) Does anyone have some explanation posted online that I could read 
to inform myself and perhaps settle my anxiety?
    (b) Is there a version of metamath and/or set.mm that don't rely on 
definitions as axioms?

    Terribly sorry if this was discussed multiple times (it must have), but 
a quick search in this mailing list didn't match any obvious prior 
discussion, so apologies if I missed some.

    Sam

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