Section 299 IPC: Abetment of Murder - Conflicting Logic

I'm totally lost on this one. Section 299 IPC states that abetment of murder is punishable when an attempt to murder is abetted, even though the murder itself doesn't happen. But then, what's with the clause that says the abettor must know that a murder will be caused? It's like, how can you know it's murder if the attempt fails? If the person I'm abetting tries to kill but misses, is it still murder? Or is it attempted murder? Does this make the abettor guilty of murder or just attempted murder? This has been driving me crazy. I've read it a dozen times, but I just can't wrap my head around it. Help, someone please explain this paradox to me.