A comforting line is making the rounds again: A-share average P/E ratios have fallen to historic lows, book values sit near their floor, and so the conclusion writes itself — "opportunity now outweighs risk," "the bull market is at the door." The seduction of this logic is that it reduces investing to a single sum: the thing got cheap, therefore you should buy it. As if a low valuation were, by itself, a promissory note with a date already written on it.

It sounds self-evidently true. Buy the cheap thing, sell the dear one — that is nearly everyone's first instinct about investing. And yet it is precisely that first instinct that has buried the deepest losses of retail investors across decades. Because it quietly folds two entirely different things into one: a thing being cheap, and that thing going up, are separated not by a pane of glass but by a canyon.