Human beings shake on the edge of high precipices. As I once did in Montana, climbing up to the top of a ridge in the Crazy Mountains whose edge was razor-sharp. Peering down into the abyss on the other side, I began to shake. I wasn't just trembling, nor was I shuddering, I was actively shaking. My body shook with fear — fear of falling into the vast vale that yawned before me and below me. I was fearful of losing my balance, and so falling into the space below. My shaking, though immediate and involuntary, itself contributed to the likelihood that I would lose my balance and fall face forward. I became dizzy, a classical symptom of vertigo. In order to avoid this consequence, I had to draw away from the edge — turn around and crawl back down the face of the ridge I had climbed so confidently. My companions, seasoned mountain climbers, stood their ground at the top, smiling at my retreat with barely concealed contempt.