 For Natorp, as for Kant, thinking is activity, doing: the technical term is “function”. This word, however, is as ambiguous as it is central to the Marburg account of  thinking.[25]  Occasionally, “function” seems to mean something like subjective, psychic act, and as such is excluded from epistemological consideration (Natorp 1887: 266); usually, however, it signifies the spontaneity of thinking, not in psychological terms, but as the rational act of hypothetical legislation discussed earlier. In this second signification it is sometimes connected with “function” in the mathematical sense, or else mathematical functions are adduced as paradigmatic of the functional conception of  thinking.[26]  The standard sense of “function” for Natorp, then, is that of an act or “operation” of thinking, where this means laying down a hypothesis. Since the hypothesis is always a concept, a generality that imposes a unity upon a phenomenal manifold, we can understand “function” more precisely as the act of unification or synthesis. 