The existence of free will and its prerequisites have entertained countless debates. The position one takes in these debates dictates their outlook on limitations of knowledge. Like the majority of philosophers and researchers, Harris (2012), in his book, misses out on exploring the consequences accepting or rejecting free will has for the predictability of human psychology. In the present review, I show the following: If we understand and model human cognition and behavior as a result of environmental & individual causes that both are knowable, adding a per-definition indeterminable source, namely, free will, can only reduce the maximum power of our prediction model. That is, accepting a free human will renders impeccable foresight theoretically, and empirically, impossible. I situate its consequences in the three exemplary application fields of management and leadership, historic analysis, and theory building.