 Suppose that, in accordance with the possible-worlds framework for characterizing modal statements (statements about what is possible or necessary, about what might or could have been the case, what could not have been otherwise, and so on), we treat the general statement that there might have been purple cows as equivalent to the statement that there is some possible world in which there are purple cows, and the general statement that there could not have been round squares (i.e., that it is necessary that there are none) as equivalent to the statement that there is no possible world in which there are round squares.