### Disjunction

February 15, 2021

Some religions see enlightenment in contemplation of the navel, but mathematicians have been fascinated by something of analogous shape, the number zero (0). The ancient Greeks had a problem with the concept of zero (μηδέν, minden), and they had no symbol for it. Their argument was, "How can nothing be something?" They had similar problems with the idea of a vacuum. The paradoxes of Zeno of Elea (c.495-c.430 BC) arose from the idea that a zero quantity was unattainable (see figure).

In Zeno's Paradox of Achilles and the Tortoise, Achilles can never outrun a tortoise having a head start, since he must always reach the point where the tortoise once was, but departed, with the consequence that the tortoise can never be overtaken. This paradox expresses the idea that the distance between Achilles and the tortoise can never be zero; and, therefore, zero can't be transcended to change the sign of that distance. This was my favorite paradox when I was a high school student. (Modified Wikimedia Commons image by Daniele Pugliesi. Click for larger image.)

Moving ahead into Anno Domini times, Ptolemy (c.100-c.170) was using a symbol for zero, a small circle with a line above it, around 150 AD in his Almagest. Zero in Latin texts throughout the Middle Ages was much different, being essentially the letter, N, used as shorthand for nulla (none) or nihil (nothing). By the start of the 18th century, the use of our present symbol for zero was firmly entrenched in Latin. It was used by Leonhard Euler (1707-1783), who wrote many mathematics papers in Latin, as the following example illustrates.

Proposition 4 from De summatione innumerabilium progressionum (1738) by Leonhard Euler, illustrating the use of the symbol for zero. In translation it reads, "§4. This form is assumed as the general term of a certain progression: ∫((1-xn)/(1-x))dx; which clearly integrated, thus so that it becomes equal to zero if x = 0, and on putting x = 1, the term of order n is given." Latin text from Ref. 1, and English translation from Ref. 2.[1-2]

All people are familiar with the basic mathematical symbols for addition (+), subtraction (−), multiplication (×), division (÷), and the square root (√), and most technologists know also the symbols for summation (∑) and integration (∫), as well as those for comparison; i.e., less than (<), greater than (>), less than or equal (≤), and greater than or equal (≥). Lesser known are the basic symbols of symbolic logic. Two of these are contortions of Roman alphabet letters; namely, the existential quantification operator (∃, normally expressed as "there exists," as in ∃(x), "there exists an x"), and the universal quantification operator (∀, "for all." as in ∀(x), "for all x").

Other symbols in mathematical logic are the symbols for "and" (⋀), "or" (⋁), and also material conditional (⊃, also written as →) that's used in statements such as modus ponens; viz,
If P, then Q (P⊃Q)
P is true
Therefore, Q is true.

You can view many unicode characters in the category, "Math Symbol," at Ref. 3.[3]

Landon D. C. Elkind of the University of Alberta (Edmonton, Alberta, Canada), and Richard Zach of the University of Calgary (Calgary, Alberta, Canada) have recently published a lengthy paper on arXiv in which they've investigated the origin of the symbol for logical disjunction ("or," ⋁).[4] It has been suggested by many historians that, just as N was chosen to represent zero from the first letter of the Latin words that express nothingness, ⋁ merely derives from the first letter of the Latin word for or; namely, vel.[4]

Leibniz had used the letter v with an accent mark called the inverted breve, ͡v, as the disjunction symbol. Peano remarked that this symbol without the accent is a sensible choice because v is the initial letter of vel.[4]

Excerpt from Principia Mathematica by Alfred North Whitehead (1861-1947) and Bertrand Russell (1872-1970) showing use of the disjunction symbol. (Portion of a scan of page 13 of my copy.[5])

While Peano made a case for using a v for disjunction, he didn't follow his own advice. Instead, he used the union symbol, ∪, from set theory.[4] Elkind and Zach conclude that Bertrand Russell's use of the symbol is what codified its place in symbolic logic.[4] Russell tended to follow Peano's lead in notation, and Whitehead and Russell's Principia Mathematica was influential in mathematical logic.[4] As Elkind and Zach write,
"The appealing short story told that our '∨' for disjunction comes from the Latin vel and is due to Leibniz is not well-supported by the textual record. It was Russell, not Leibniz, who first systematically used ‘∨’ for disjunction, and it is more likely given Russell’s logical habits that this choice of notation was to stress the analogy of propositional addition with class union."[4]

### References:

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