  In the early stages of everyday knowing, what is re-discovered or recognized when something is known is an intuitive idea.  Intuitive ideas present images which are signs of their contents and are drawn from sensory experience. Of course, images are vague, blurred, and ill-defined so that when one conjures up an image, say, of one’s father, the expression on his face may not be clear and distinct, so that it may be impossible to tell whether he is frowning or merely looking puzzled (Schlick 1979a, 126–7; Schlick 2009, Sec.  4). And while intuitive ideas are sufficient for the purposes of everyday life, scientific inquiry naturally demands more rigorous methods for capturing and expressing ideas. For this reason, concepts – ideas with precisely delineated contents – are used. And while the meanings of terms used in everyday discourse are usually intuitive ideas, in science they are almost exclusively concepts. This provides scientific judgments with content which is accurately circumscribed, while at the same time eliminating their intuitive content. In his General Theory of Knowledge of 1918, Schlick explained that concepts are formed in clusters, just as the primitive concepts of a mathematical field are defined in terms of one another by the axioms of the discipline. But in his earlier epistemological writings, he explains concept-formation in a more traditional fashion, by reference to marks or characteristics (Merkmale) which belong to all the objects which fall under the concept. Concepts thus represent classes of objects, defined in terms of determinate traits, so that their scope is exactly demarcated. Thus they differ from intuitions, which are indistinct representations of what is presented to a particular sensory modality. So the intuition of a triangle in general or a man in general can only be a hazy, fuzzy-edged visual representation of some particular triangle or man. And while everyday knowing proceeds by comparisons of intuitions, scientific knowledge replaces intuitions in these comparisons with precisely delineated concepts. In short, it is through its reliance on concepts that scientific thinking takes knowledge to a higher level than everyday knowing.