What makes a theory good? In his canonical 1991 book Inference to the Best Explanation, Peter Lipton attempts to answer this fraught question. The philosopher identifies eleven explanatory virtues that are often placed within four groupings: evidential, coherential, aesthetic, and diachronic. Two others, James Beebe1 and Kenneth Dillon2, draw upon the same categorical schema to present four other virtues for consideration. All fifteen are listed and defined in the following table: