Among philosophers, he was for a time best known for his interpretation of Descartes's rationalism. His most influential work, however, has been on freedom of the will (on which he has written numerous important papers) based on his concept of higher-order volitions and for developing what are known as "Frankfurt cases" or "Frankfurt counterexamples" (i.e., thought experiments designed to show the possibility of situations in which a person could not have done other than he/she did, but in which our intuition is to say nonetheless that this feature of the situation does not prevent that person from being morally responsible). Frankfurt is probably the leading living Humean compatibilist, developing Hume's view that to be free is to do what one wants to do. (Others who develop this view are David Velleman, Gary Watson and John Martin Fischer.) Frankfurt's version of compatibilism is the subject of a substantial literature by other philosophy professors. More recently, he has written on love and caring.