## Numbers & Quantities

Numbers are mathematical objects used to count, measure, and label. They can be represented with words or by symbols called numerals.

As only a small number of symbols can be memorized, numerals are commonly organized in a numeral system. The most common one is the Hindu–Arabic decimal system, which allows for the representation of any number using a combination of ten fundamental numeric symbols, called digits (from 0 to 9).

Prehistoric tally sticks, a unary numeral system – like finger-counting – are believed to be one of the first tools used for counting elapsed time, such as the number of days, of lunar cycles, or for keeping records of quantities.

Natural numbers are those used for counting and ordering: 1, 2, 3, 4, and so forth. Between the still unknown moment when mankind invented numbers and today, the notion of number has been extended to include 0, negative numbers, fractions, values of continuous physical quantities, etc.

Among this infinity of numbers expressed in our decimal numeral system, some stand out from others, play a key role, or appear curiously often in many fields such as the golden ratio $$\phi$$, the number pi $$\pi$$, or the Fibonacci numbers.

Counting was probably one of the primary skills early humans had to handle. This ability has even been observed in various animal species, from fishes to primates. Today, it is impossible to imagine a world without numbers. They are omnipresent, inside us, around us, and far beyond.