According to Aristotle, what is common to all emotions is the presence of pleasure or pain. He says, 'By passions I mean appetite, anger, fear, confidence, envy, joy, friendly feeling, hatred, longing, emulation, pity, and in general the feelings that are accompanied by pleasure or pain ... (EN II 5; 1105 b 2123, emphasis added). In this definition Aristotle has certainly pointed to one essential feature of those mental phenomena that we call emotions, for it is impossible to think of an emotion that would not be coloured by the feelings of pleasure and unpleasure. (We prefer the modest word 'unpleasure' to the excessively strong 'pain'.)
Brentano adopts a basically Aristotelian conception of the emotions when he says that the defining characteristic of the emotions (which he also calls the 'phenomena of love and hate') is the presence of an attitude of either acceptance as good or rejection as bad that is directed to the object of the act. In spite of apparent dissimilarity, Brentano's view is, if not identical with, at least compatible with Aristotle's, for we have naturally a positive attitude towards pleasure and negative attitude towards unpleasure or pain.