Junior citizens, this is an antique calculating device called a slide rule. On its fixed D scale and sliding C scale (click to enlarge) it physically represented multiplication and division by adding or subtracting measured lengths proportional to the values of the terms’ logarithms:

log (ab) = log a + log b

Length proportional to product = sum of lengths measured on scales C and D

In the notation for a base-10 logarithm, the digit to the left of the decimal point is an integral exponent of 10, with (for instance) 2 representing 10 squared, or 100. Likewise, 3 represents 10 cubed, or 1000, and a number larger than 2 but smaller than 3 represents a value between 100 and 1000 such as 500, whose logarithm is 2.69897.

But nothing to the left of the decimal point appears on the slide rule. There, the digit 5 is a right-side value only, understood to represent symbol as such, stripped of any idea of quantity. It may stand for 5 or 500 or 0.0005, but to learn which you’ll have to supply the zeroes yourself, filling them in from mind. Zero’s only domain is mind. It isn’t to be found among the physical symbols of quantity cycling from 1-on-the-left to 1-on-the-right along C and D. Zero doesn’t come to mind through the senses.

But quantity does. It is of the body. On a slide rule you can perceive it through the cursor’s body-warm glass. And see: even when the plenum beyond rule goes empty, what remains to be seen is not nothing.

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