How do psychological factors influence our ability to push the limits of our physiological capabilities? What role does the brain play in protecting us from pushing too far? How does nutrition, thirst, heat, pain tolerance, etc. contribute to our ability to endure and get the most out of our bodies? These are the questions the author explores, heavily researched and in a highly structured way. Most answers – even with loads of data analysis at his side – end up being “somewhat” or “possibly” or “to an extent” or “we still aren’t sure.” The smart way the author went about it didn’t make those gray answers frustrating. It wasn’t light and breezy reading, yet I mostly enjoyed it and found several techniques intriguing to try and deploy in my own endurance efforts.