Sfard (1991) takes a process/object view of the development of complex number, similar to the way in which other numbers were developed. For example, when subtracting, say, five from three, the process — subtraction — led to the development of an object — negative
number. Each expansion of our concept of number can be explained in a similar manner, with division motivating the need for rationals, taking square roots motivating the need for irrationals, and, later, the taking of roots of negative numbers motivating the need for complex numbers.