In there stepped a stately Raven of the saintly days of yore. Not the least obeisance made he; not a minute stopped or stayed he; But, with mien of lord or lady, perched above my chamber door— Perched ...

If you were to find a nonblack raven sitting outside your chamber door, the statement would be disproved. But you can’t possibly observe all ravens, so the German philosopher Carl Gustav Hempel ...