For most of the twentieth century, manufacturers treated inventory as an unambiguous asset: warehouses stocked with finished goods and raw materials were a visible measure of a firm's readiness to meet demand. The just-in-time philosophy, imported from Japanese automakers, reframed that view sharply. Under just-in-time thinking, inventory is not a buffer but a liability—idle capital tied up in goods that earn nothing while they sit, that occupy costly warehouse space, that risk obsolescence, and that, by their very presence, conceal underlying problems in production scheduling and quality control. The leaner the inventory, this view holds, the more sharply such problems reveal themselves and the more pressure there is to solve them at the root.

Yet the philosophy carries a cost that its enthusiasts sometimes understate. Safety stock exists precisely because demand and supply are uncertain. A firm that holds little or no buffer is exquisitely sensitive to disruption: a single late shipment, a labor stoppage at a supplier, or an unexpected demand spike can halt production or empty shelves, and a stockout is not merely a deferred sale but often a permanently lost customer who turns to a competitor. The carrying cost of inventory is real and measurable; the cost of a stockout is real but harder to quantify, and the difficulty of measuring it tempts managers to discount it.

The more sophisticated practitioners therefore reject the framing of inventory as simply 'good' or 'bad.' They treat the optimal buffer as the solution to a trade-off, balancing the carrying cost of holding stock against the expected cost of the stockouts that stock prevents—a balance that shifts with the volatility of demand, the reliability of suppliers, and the margin lost when a sale is forgone. By this account, the lean-inventory revolution was not the discovery that inventory is waste, but the discovery that firms had been systematically overweighting one side of a calculation they had never explicitly performed.