Typically, those assumptions involve matters such as the following: that stocks and other financial assets can be bought and sold at prevailing market prices without affecting those prices, that no commissions or other "transaction costs" are incurred in so doing, that stocks can be "sold short" (e.g., borrowed and sold, and later repurchased and returned) freely and without penalty, and that money can be borrowed and can be lent at the same "riskless" rate of interest.

Donald MacKenzie, An Engine, Not a Camera, loc. 51