His research into the innate capacity for empathy among primates has led De Waal to the conclusion that non-human great apes and humans are simply different types of apes, and that empathic and cooperative tendencies are continuous between these species. His belief is illustrated in the following quote from "The Age of Empathy": "We start out postulating sharp boundaries, such as between humans and apes, or between apes and monkeys, but are in fact dealing with sand castles that lose much of their structure when the sea of knowledge washes over them. They turn into hills, leveled ever more, until we are back to where evolutionary theory always leads us: a gently sloping beach."