This splendid book began in December 2013, over a cup of tea at my friend Rustam Atadjanov’s apartment in Tashkent. At the time, both of us were legal advisers at the ICRC Regional Delegation in Central Asia, I just defended my Dr. iur. thesis on the crime of aggression in international criminal law (ICL),1 and Rustam was contemplating a doctorate. As we were discussing gaps in ICL, in search for a possible thesis topic for Rustam, there emerged a number of questions: what exactly is this “humanity” against which “crimes against humanity” are directed? Why include a philosophical notion in a central concept of ICL, which must be sufficiently specific, by virtue of the principles of legality and legal certainty? What to do about the multiplicity of meanings of the word “humanity”? Which of those distinct meanings is implied in the concept of crimes against humanity? Why did the authors of the term not opt for an alternative, more “measurable” term—for example, “crimes against the civilian population”? What is the relationship between “crimes against humanity” and “the laws of humanity” referred to in the Martens Clause? As the discussion progressed, it became quite clear that Rustam did identify a topic for his future thesis. Next year, both of us left the ICRC, with an interval of four months: I assumed my academic position at KIMEP University in Almaty, and Rustam embarked, with his natural curiosity and hard-working attitude, on a challenging academic journey, which would result in this book.