Chisholm was dubious about such things as sense data, appearances, the “looks” of things. He defended a different view about the ontology of perception. An amusing early paper on this topic is “The Problem of the Speckled Hen” (1942). A more extended discussion, including a presentation of Chisholm’s preferred theory of appearing, can be found in Chapter 8 (“Sensing”) of [PPS]. More than forty years later, near the end of his career, Chisholm made some final remarks about appearances in Chapter 13 of [RTC].