IT is rather curious that no European astronomer observed the supernova which occurred in our Galaxy in about 1668 and left the remnant Cassiopeia A. However, if the absolute magnitude of the supernova is Mv = -19 mag, the interstellar absorption is Av = 4.3 mag (ref. 1) and the distance is 2.8 kpc. Then its maximum visual magnitude should have been about -2.5 mag. Even ~100 d after the event, the supernova should have been as bright as any star in the Cassiopeia constellation. The fact that no such object was recorded suggests that the optical luminosity of the supernova may have been several orders of magnitude lower than the usual value. This note argues that the low luminosity of Cas A may be an indication that the product of the explosion is a black hole.