 Investigations in logic after Aristotle (from later antiquity through the twentieth century) seem to have focused exclusively on deductive logic. This continued to be true as the predicate logic was developed by Peirce, Frege, Russell, Whitehead, and others in the late nineteenth and early twentieth centuries. With the collapse of logical positivism in the mid-twentieth century (and the abandonment of attempts to treat the physical world as a logical construction from facts about sense data), new attention was given to the relationship between sense perception and the external world. Roderick Chisholm (Chisholm 1957; Chisholm 1966) argued that sensory appearances give good, but defeasible, reasons for believing in corresponding facts about the physical world. If I am “appeared to redly” (have the sensory experience as of being in the presence of something red), then, Chisholm argued, I may presume that I really am in the presence of something red. This presumption can, of course, be defeated, if, for example, I learn that my environment is relevantly abnormal (for instance, all the ambient light is red).