Abstract
The objective of this essay is to show how the concept of number ‘one’ has changed, from its initial formal definition in Euclid’s The Elements until very recently. Some of these changes are mathematical, others are philosophical, and others are historical in character. This retrospective of the concept of number ‘one’ implies that the history of mathematics is much more complex and rich than that which is suggested by our intuition when we limit ourselves to a unique, lineal and continuous mathematics.