

The impossibility of perfect forgeries? - samclemens
http://blog.oup.com/2015/02/impossibility-perfect-forgeries/

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sirclueless
I think the author is trying to say that there is a concept of knowledge that
is inherent to words "perfect forgery". That either someone knows the origin
of an object and therefore it is not a perfect forgery, or all such knowledge
has been erased and now there is no way to say that an object is not
authentic.

I think this is something of a straw man argument. There is an entirely
satisfactory functional definition for "perfect forgery" which is that an
object is a perfect forgery if it cannot be distinguished from an authentic
object by someone with no prior knowledge of the object's origin. This is what
I believe common intuition would determine the word "forgery" to mean. An
object does not fail to be a perfect forgery because its creator knows it is
not real. It is a perfect forgery if it can fool everyone else into believing
it is real.

Making zero knowledge of authenticity a prerequisite for a "perfect forgery"
is to neglect that there are multiple independent minds in the world with
their own viewpoints. I can _know_ "that is a perfect forgery" if I know it
will fool anyone without my private knowledge. Such knowledge is not
incompatible with the object being a perfect forgery, because the most
sensible definition of "perfect forgery" is that it would fool anyone else,
not that it has already fooled everyone including me.

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Reebles
When we talk about whether something is real or a forgery, I don't think we
really care _what_ it is composed of, we care about its provenance. The
composition is really just a shortcut often used to help us identify obviously
fake provenance; if something is not composed of the same stuff as the real
thing, well then we can disqualify it from being real. I don't think anyone
should suppose that identical composition is sufficient to guarantee real
provenance though.

For perfect forgery detection, I think you need a complete and unimpeachable
record of where each bill has been at each moment of its existence. The
current physical properties of the item are just not enough. Obviously that is
infeasible, though I guess crypto-currencies basically do that.

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evincarofautumn
Isn’t this just saying that not all proofs are constructive? That is, just
because you can prove the existence of an object doesn’t mean you necessarily
have a method for identifying it.

Also, there is some vague appeal to a substructural logic here, since by
mixing up the forgery with the genuine article, you’re destroying the
information about their origins, which is what prevents you from identifying
the forgery afterward.

I’m not up on my modal logic, but I think this can be expressed as ◻∃x.P(x) ↛
∃x.◻P(x)—proving the existence of some x for which P is true does not imply
that there exists any particular x for which P is provable.

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twiss
In other words, to have a perfect forgery you must not know it is a forgery.
If you could screen for knowing that a bill is a forgery, it would be useless
to make one (since you could not spend it, since you knew it was a forgery).
Bummer we can't. /s

