One may say that The Open society and Its Enemies (OS) offered in 1945 the first complete elaboration of the general approach proposed by Karl Popper, namely his ‘critical rationalism’, a bold generalization of the fallibilist falsificationism in the domain of the empirical sciences masterly proposed in Logik der Forschung (1934). The political content of The OS has been critically discussed. Nevertheless, not all people insist on the equally important moral dimension of the book, giving it its unity, I submit. Without morality, no critical discussion, no reason, no open society, let us say in a nutshell. I would argue that according to Popper, a strictly Christian morality of love would not be the appropriate emotional companion of critical rationalism, but that the less demanding moral emotion of sympathy or compassion is perhaps necessary to give it its force against violence. I give some support to this line of argument. In my view, Popper proposed a somewhat unarticulated critical rationalist ‘emotivism’ of sorts. The emotion of compassion is necessary for triggering our moral decisions and values, which are the ultimate basis of the choice for a reason against violence.