YOU QUADRUPLE FOOL! You do not understand a person in the past because you cannot experience the socio-cultural aspects of the past! Sources are relied upon for context and content!
Well, no. But through the study of history you can gain greater understanding of what drove people in the past, a lot of what would motivate them may not motivate people in the modern world, but you may see similarities between how people then reacted to an event and how people in the modern world react to a similar situation. But people studying the past inevitably study the people in the past, so I'd say it's wrong to say that only scientists in the fields of psychology, biology and sociology study human being - one that frequently comes up in lectures on events such as the crusades is "Would we do this again?" and while most people would say no, one of my lecturers was quick to point out that people were quick to volunteer in WWI and showed a spirit similar to that was shown at the start of the First Crusade.
I'd also point out that Williams' isn't claiming that the study of history gives one the monopoly on empathy, although there will be empathy amongst those studying, for example if you study an event like, for example, the sinking of the
RMS Titanic, then you certainly feel a lot of sympathy for those involved. That doesn't mean you have a monopoly on empathy, but (from my own point of view) you'd need a heart of stone not to feel moved by the stories.
One other thing - you say that this book is required reading at many colleges and universities. Which ones? I've studied history at the Aberdeen University, Stirling University and am now doing a history course with the Open University, so far this book hasn't come up. What does come up, time and time again, is the need to be analytical of sources and the need to check the veracity of sources, but I've yet to see this book anywhere in my (exhaustive) reading lists.