In Martin-Löf (1996) a general philosophy of logic is presented where the traditional notion of judgment is expanded and given a central position. A judgment is no longer just an affirmation or denial of a proposition, but a general act of knowledge. When reasoning mathematically we make judgments about mathematical objects. One form of judgment is to state that some mathematical statement is true. Another form of judgment is to state that something is a mathematical object, for example a set. The logical rules give methods for producing correct judgments from earlier judgments. The judgments obtained by such rules can be presented in tree form