"Rather suddenly, it came to us," Black later recalled. "If the stock had an expected return equal to the [riskless] interest rate, so would the option. After all, if all the stock's risk could be diversified away, so could all the option's risk. If the beta of the stock were zero, the beta of the option would have to be zero too.... The discount rate that would take us from the option's expected future value to its present value would always be the [riskless] interest rate."

Donald MacKenzie, An Engine, Not a Camera, loc. 583