 A prominent defender of dispositionalism is Johnston (1992), whose account of the major constraints upon a theory of color we examined in an earlier section. He concedes that dispositionalism has difficulty handling the constraint imposed by commitment to the doctrine of Revelation but, he thinks, giving this up is a small price to pay. He maintains that, nevertheless, the theory can handle all the other constraints, and in so doing has major advantage over rival accounts. One of the theory’s merits is that it can account for the Principle of Unity, item (3) in his list, although as we have seen, this principle needs to be extended. Another merit of Johnston’s version of dispositionalism is that it handles what seems to be a difficulty for other versions, the problem of explaining the causal role of color in the perception of colors, that is to say, the problem of meeting constraint (2) on his list: