I … do not direct myself so badly. If it looks ugly on the right, I take the left … Have I left something unseen behind me? I go back; it is still on my road. I trace no fixed line, either straight or crooked.
—Michel de Montaigne (1533–1592), The Essays
It is time to admit that we will not be able to completely fulfill the promise contained in this book's subtitle—explain how humans plan their motion. This would be good to do—such knowledge would help us in many areas—but we are not in a position to do so. Today we know precious little about how human motion decision-making works, certainly not on the level of algorithmic detail comparable to what we know about robot motion planning. To be sure, in the literature on psychophysical and cognitive science analysis of human motor skills one will find speculations about the nature of human motion planning strategies. One can even come up with experimental tests designed to elucidate such strategies. The fact is, however, that the sum of this knowledge tells us only what those human strategies might be, not what they are.
Whatever those unknown strategies that humans use to move around, we can, however, study those strategies' performance. By using special tests, adhering to carefully calibrated test protocols designed to elucidate the right questions, and by carrying out ...