 Let’s revisit the classic question of whether all ravens are black. Imagine an ornithologist who tackles this problem by examining one raven after another. There is exactly one observation sequence in which only black ravens are found; all others feature at least one nonblack raven. The figure below illustrates the possible observation sequences. Dots in the figure denote points at which an observation may be made. A black bird to the left of a dot indicates that at this stage, a black raven is observed. Similarly, a white bird to the right of a dot indicates that a nonblack raven is observed. Given a  complete sequence  of observations, either all observed ravens are black or nonblack; the figure labels complete observation sequences with the statement that is true of them. The gray fan indicates that after the observation of a white raven, the claim that not all ravens are black holds on all observation sequences resulting from further observations.