A Spark by Innocent Mwatsikesimbe from Questions Asked, chapter 8

Both go by the same name — plan B — which is what makes them dangerous. A backup is another route to the same destination: if one way is blocked, the dream still gets reached. A fallback faces the other way entirely; it’s a home being built in the world where the dream has already failed.

Innocent doesn’t forbid honesty about failure. You will have to decide, he says, at what point you’d call the venture unsuccessful and move on. What he warns against is the disguise — fallbacks dressed up as backups — because a fallback can “demotivate you and stop you from trying your best, if you give it too much importance and attention.” Part of you moves into the failure early, and the trying gets done by whatever is left.

So the test of any plan B is direction, not content. Does it exist so the dream succeeds, or does it exist for afterward? One deserves work. The other deserves, at most, a page.