**by Paul Curzon, Queen Mary University of London**

*Binary is at the core of the digital world, underpinning everything computers do. The mathematics behind binary numbers was worked out by the great German mathematician Gottfried Wilhelm Leibniz in the 17th century. He even imagined a computing machine a century before Babbage, and two centuries before the first actual computers. He was driven in part by an ancient Chinese text used for divination: predicting the future.*

## I Ching

Leibniz was interested in the ancient Chinese text, the I Ching because he noticed it contained an intriguing mathematical pattern. The pattern was in the set of 64 symbols it used for predicting the future. The pattern corresponded to the counting sequence we now know of as binary numbers (see** I Ching binary**). Leibniz was obviously intrigued by the patterns as a sequence of numbers. Already an admirer of the great Chinese philosopher, Confucius, he thought that the I Ching showed that Chinese philosophers had long ago thought through the same ideas he was working on. Building on the work of others who had explored non-decimal mathematics, he worked out the maths of how to do calculations with binary: addition, subtraction, multiplication, and division as well as logical operations such as ‘and’, ‘or’ and ‘not’ that underpin modern computers.

## Algorithms embedded in machines

Having worked out the mathematics and algorithms for doing arithmetic using binary, he then went further. He imagined how machines might use binary to do calculations. He also created very successful gadgets for doing arithmetic in decimal, but saw the potential of the simplicity of the binary system for machines. He realised that binary numbers could easily be represented physically as patterns of marbles and the absence of marbles. These marbles could flow along channels under the power of gravity round a machine that manipulated them following the maths he had worked out.

A container would hold marbles at the top of a machine. Then, by opening holes in its base above different channels, a binary number could be input into the machine. An open hole corresponded to a 1 (so a marble released) and a closed hole corresponded to a 0 (no marble). The marbles rolled down the channels with each channel corresponding to a column in a binary number. The marbles travelled to different parts of the machine where they could be manipulated to do arithmetic. They would only move from one column to another as a result of calculations, such as carry operations.

Addition of digits in binary is fairly simple: 0+0 = 0 (if no marbles arrive then none leave); 0+1 = 1+0 = 1 (if only one marble arrives then one marble leaves; (1+1 = 2 = 10) if two marbles arrive in a channel then none leave that channel, but one is passed to the next channel (a carry operation). The first rules are trivial, open the holes and either a marble will or will not continue. Adding two ones is a little more difficult but Leibniz envisioned a gadget that did this, so that whenever two marbles arrived in a channel together, one was discarded, but the other passed to the adjacent channel. By inputting two binary numbers into the same set of channels connected to a mechanical gadget doing this addition on each channel, the number emerging is a new binary number corresponding to the sum.

Multiplication by two can be done by shifting the tray holding the number along a place to the left. In decimal, to multiply a number by ten, we just put a 0 on the end. This is equivalent to shifting the number to the left: 123 becomes 1230. In binary the same thing happens when we multiply by 2: put a 0 on the end so shift to the left and the binary number is twice as big (11 meaning 2+1 = 3 becomes 110 meaning 4+2+0 = 6). Multiplication of two numbers could therefore be done by a combination of repeated shifts of the marbles of one number, releasing copies of it or not based on the positions of the 1s in the other number. The series of shifted numbers were then just added together. This is just a simplified version of how we do long multiplication.

To multiply 110 by 101, you multiply 110 by each digit of 101 in turn. This just involves shifts, and then discarding or keeping numbers. First multiply 110 by 1 (the ones digit of 101) giving 110. Write it down. Now shift 110 one place to give 1100 and multiply by 0 (the twos digit of 101). This just gives 0000. Write it below the previous number. Shift 110 by another place to give 11000. Multiply it by 1 (the fours digit of 101). That gives 11000. Write it down below the others. Add all three numbers to get the answer (see box).

## The basics of a computer

Leibniz had not only worked out binary arithmetic, the basis of a computer’s arithmetic-logical unit (ALU), he had also seen how binary numbers could flow around a machine doing calculations. Our computers use pulses of electricity instead of marbles, but the basic principles he imagined are pretty close to how modern computers work: binary numbers being manipulated as they flow from one computational unit to the next. Leibniz didn’t make his machine, it was more a thought experiment. However, helped by I Ching, a book for divining the future, he did essentially predict how future computers would work.

## More on …

- I Ching binary
- Cunning Computational Contraptions

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This article was funded by UKRI, through Professor Ursula Martin’s grant EP/K040251/2 and grant EP/W033615/1.