A paper about vagueness in paraconsistent logic (Weber, Z. 2010) gave a rhetorical example of the interaction between vague terms and real analysis. Imagine walking down the largest mountain in the known world. Olympus Mons on Mars has a very gentle slope or declivity, so that you can start by describing yourself as “high up” but as you walk down you become unsure if you are “high up” or “not high up”. This is graphable on an xy coordinate system with 0 at the top of the mountain and 1 at the foot of the mountain. The interval of “high up” points has a least upper bound (of x values) called “sup” and the interval of “not high up” points has a greatest lower bound called “inf.” Now we will suppose, as this is a presentation of vagueness, that “sup” is also “high up” (which is reasonable since all the points less than it are also high up) likewise “inf” is “not high up”. We encounter a contradiction in all three possibilities: If “sup”=”inf” then “sup” is both “high up” and “not high up” if “sup” > “inf” then by density of the reals “sup” > z > “inf” with z both “high up” and “not high up”, likewise if “inf” > “sup”.

Now we can put the blame on the vague term “high up” which is clearly not very technical, and go on to fantasize a perfectly precise world of mathematics that should not be sullied by vague words, but such a world is more difficult to defend than the fantasy suggests. First of all, mathematical equations with no tie to real world meanings are widely regarded as ambiguous, a term usually distinguished from vagueness. Ambiguous means there is more than one possible meaning or interpretation. Hesse points out that Socrates’ famous straight stick in water, the apparent bend in the stick was meant to represent falsehood. Now the bend can be represented with an equation involving the theory of refraction:

“sin(alpha)/sin(beta) = Mu”

can be interpreted in other ways besides that alpha and beta are angles and Mu is a constant about air and water— “They might, for example, be the angles between the Pole star and Mars and Venus respectively at midnight on certain given dates; why would not this be a confirmation of the formalism we have mistakenly called the wave theory of light?…” (Hesse as cited Structure of Scientific Theories Suppe 1977, p 100-101)

Now suppose I invent a word that means ambiguously “tall” and “not tall.” Similar words are found in language, for example the Thai “Krup” means “yes” but also does not mean yes. It is used because yes is too strong and seen as insulting a person’s intelligence, and “krup” rather is a polite sound indicating the speakers faith that the listener can figure it out for themselves. I could invent a word “snook” that can mean “tall” and “not tall” in different situations. Is the word ambiguous when it is applied to the one of these borderline cases when “tall” is vague? or does the ambiguity of the word capture the vagueness of the phenomena? Is this a vagueness between ambiguity and vagueness? What does that mean for ambiguity in mathematical equations?

The mathematical part of the wave theory of light, if it is to correspond to reality at all, is vague. Even if we were to abandon mathematical application to science, pure mathematics still uses words such as “continuity” “completeness” and “integral” which are vague notions. In fact, a standard text in analysis will show how mathematical definitions fail to perfectly capture the idea of continuity, with fuzziness in functions getting past the definition, and being allowed to be called technically “continuous”. Without this sparse collection of vague words, math texts would be hopelessly meaningless. The vagueness of the words continuity and completeness are in fact very important to being able to learn and understand analysis.

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