[yobbo]

Predictability is defined in terms of probability distributions, and a price X is typically defined to be drawn from something like Xₜ₊₁ ~ Xₜ + N(μ,σ). The purpose might be to quantify some property of μ or σ or something like that. It means "X₀.ₜ does not hold any information about Xₜ₊₁". The assumption is that any price move reflects new information. It might be a reasonable model for various purposes.

But if this model was "true", RenTec would not work, and the "efficient market hypothesis" is invalidated. Which seems plainly obvious.

Ok, so if the market is not efficient, then it's actually a game (poker-like?) and zero sum. Academically, unthinkable thoughts.