This paper examines the fundamental nature of numbers as they are used in economic systems. In the framework proposed, number sequences are technological objects (‘tools’) that are constituted by both form and function. To do their job, number sequences have to have the necessary internal structure – all elements (e.g. symbols) of the sequence must be distinct from one another, and the sequence must be a progression. In addition, numerical toolkits have to have the right external structure – they must be situated in a social network of economic agents that confers on them quantitative functions (e.g. identifying set sizes). Number sequences are the product of multilevel evolutionary processes, including psychological selection that screens sequences for their learnability by human users. Number tools are a kind of capital; they are material systems that are as real as other everyday objects. Just as changing physical tools alters the structure of productive activity, so too changing number sequences alters cognitive, behavioral, and social routines.