A purported psychological fallacy wherein people confronted with an incongruent proposition X will say "Oh, not-X because Y", and perhaps a week later, say, "Oh, not-X because Z." Ephemeral Premises is when you make up assumptions to deflect X, don't write them down or list them anywhere, and since there's no central list, you might be making up different assumptions when talking to someone else. It's a disposable premise being used as a parry in short-term combat, not part of a long-term edifice being made with consistent, well-defended materials. Needs to be paired with understanding of the Multiple-Stage Fallacy so that you don't fall for the reverse fallacy of making people list out lots of Ys and Zs to make arbitrary propositions sound very improbable.