 The so-called “colour-exclusion problem” is a difficulty that arises for the Tractatus’s view that it is metaphysically possible for each elementary proposition to be true or false regardless of the truth or falsity of the others (4.211). In view of its generality, the problem might more accurately be termed “the problem of the manifest incompatibility of apparently unanalysable statements.” The problem may be illustrated as follows: Suppose that a is a point in the visual field. Consider the propositions P: “a is blue at t” and Q: “a is red at t” (supposing “red” and “blue” to refer to determinate shades). It is clear that P and Q cannot both be true; and yet, on the face of it, it seems that this incompatibility (or “exclusion” in Wittgenstein’s parlance) is not a logical impossibility. In the Tractatus Wittgenstein’s response was to treat the problem as merely apparent. He supposed that in such cases further analysis would reveal the incompatibility to be logical in nature: