  I want to combine two hitherto largely independent research projects,
scientific understanding and mechanistic explanations. Understanding is not
only achieved by answering why-questions, that is, by providing scientific
explanations, but also by answering what-questions, that is, by providing what
I call scientific descriptions. Based on this distinction, I develop three
forms of understanding: understanding-what, understanding-why, and
understanding-how. I argue that understanding-how is a particularly deep form
of understanding, because it is based on mechanistic explanations, which answer
why something happens in virtue of what it is made of. I apply the three forms
of understanding to two case studies: first, to the historical development of
thermodynamics and, second, to the differences between the Clausius and the
Boltzmann entropy in explaining thermodynamic processes.
