 Now if there is no paradox on the conceptual level, what then is it that makes retro-suicide physically impossible? It could be either local facts or global facts. Local facts that could constrain your action of retro-killing are many. Your hand was shaking while firing your gun, you got a fly in your eye, you were disturbed by a cat, you just fainted, etc. These constraining facts seem reasonable by themselves; they could have happened independently of your overall capacity of killing somebody in the past, but also in the actual situation interact with your ability and turn the action into an unsuccessful event. The problem is merely that such an explanation looks suspicious. It is a general fact that we cannot retro-kill anybody who is alive after the time the death purportedly took place. Likewise it is a general fact, assuming that backward causation (or time travel) is physically possible, that we can retro-kill anybody who is not alive after the time the purported death took place. But the explanation of a general fact requires an appeal to a general fact or a law of nature. Thus a reference to a singular contingent fact to explain why you never succeed in killing your younger self seems not to fulfil the requirement of being an explanation.