Let's be exceedingly clear with our definitions, instead of arguing at cross purposes. brg444 is correct when he says
Fungibility, the property of every unit in the system being indistinguishable & capable of mutual substitution, is an inherent feature of Bitcoin.
but the narrow scope of that statement is strictly limited to "the system" of Bitcoin's internal protocol.
Most people care/know far more about higher-level socioeconomic systems, wherein Bitcoin taint (distinguishability) destroys mutual substitution.
In brg444's conceptual frame, the Holy Protocol is the (sacred) ground on which socioeconomic figures stand.
That confuses those who hold the reverse POV, in which Bitcoin protocol is just a tool (figure) that does not supervene on socioeconomic ground.
Adam Back captured this linguistic tension in the concept of "weak fungibility."
https://www.youtube.com/watch?v=3dAdI3Gzodo&feature=youtu.be&t=28m31sI've proposed the term "exhaustive fungibility" to describe coins which are, by virtue of being indistinguishable, fungible at both the protocol and socioeconomic levels. But perhaps "strong fungibility" is a cleaner fit, as it complements Dr. Back's preexisting terminology.
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