  The centerpiece of Schlick’s early epistemology is a deep cleavage between intuitive acquaintance and conceptual knowledge.  Although Schlick’s distinction is reminiscent of Kant’s contrast between intuitions and concepts, Schlick regarded intuition as fully naturalized, much as Helmholtz had. When the elements of judgments are initially identified, they are grasped qualitatively, as sensory impressions, like the visual image of a particular dog or the memory image of a horse. These intuitions of acquaintance are spatially qualitative, since they are not only extended but situated with respect to one another in the space of the particular sensory modality through which they are perceived. They are also temporal, since they succeed one another in time. For an example, Schlick considered the visual image of something in the distance which, as it approaches, is identified, first of all, as an animal, then it is recognized as a dog and, when it comes close enough, it will be recognized as my dog ‘Fritz’. Each of these cases involves the recognition of one thing – the image of that which is approaching – as something else, an animal, a dog, and (finally) Fritz. Thus, each of these cases involves knowledge that the image is that of an instance of some class (Schlick 1979a, 119–121; Schlick 2009, Sec. 2). And the same process, in which one thing is recognized as another and therefore known, occurs in cases of scientific knowledge. For instance, the early explanations of light recognized that its behavior was much the same as the behavior of waves. Thus, in the work of Christian Huyghens, light came to be known as a wave phenomenon or, in other words, as the wave-like propagation of a state. Later, through the work of Heinrich Hertz, it was realized that light was unlike mechanical waves which traveled through a medium (such as water or air) but, rather, light behaved more like electrical waves. Accordingly, light became known as an electromagnetic wave phenomenon. In this case, as in the everyday case of the knowledge that the approaching animal is my dog Fritz, light was originally known as a wave phenomenon and only later did it become known as an undulatory disturbance in an electromagnetic field (Schlick 1979a, 121–2; Schlick 2009, Sec. 3).