 The Marburg philosophers characterize their philosophy as a kind of “idealism”. This idealism should be sharply distinguished from “subjectivism”, since they do not believe that objects of knowledge are representations—they do not believe that the objects of knowledge depend for their existence on the existence of minds to think them (Cassirer 1910: 298). This idealism is also opposed to the view that our knowledge is really illusory; they do not hold that there are unknowable objects, or that the real nature of objects of knowledge is in principle unknowable (Cohen 1902: 507ff). Rather, their position is idealist in four senses. First, they defend an “idealist” conception of philosophy, as following the transcendental method. In this sense, idealism is opposed to “naturalism”, the view that the method of philosophy does not differ in kind from that of the natural sciences. Second, they hold that there are substantive a priori principles that make experience possible. In this sense, their idealism is opposed to empiricism. Third, according to their theory of objective validity, our conception of an object depends on our conception of objectivity, which is grounded in the idea of the unity of knowledge according to laws. In this sense of idealism, idealism is opposed to “realism”, which takes the concept of an object as a philosophical primitive. Fourth, according to their philosophy of science (see below  section 2.3),  our empirical theories make essential use of fundamental concepts (such as mathematical concepts, or “theoretical concepts”, such as the concept &lt;force&gt; or &lt;energy&gt;) that are radically unlike the features that the world appears to have to untutored, everyday experience. These “ideal” concepts are not reducible to “sensible” concepts such as color, texture, or shape. In this sense, idealism is opposed to “sensualism”.