The Scissors

A buyer's willingness to pay is only one blade of the instrument that settles a price. Across the market stands someone who must decide whether to supply the thing, and on what terms: whether to bring the fish to the stall, make another length of cloth, or devote the factory to a different use. The preceding plates have followed the usefulness of another unit and the alternative forgone in obtaining it. Alfred Marshall brought these questions together in the image of a pair of scissors. Asking whether utility or cost governs value resembles asking which blade cuts the paper. We can hold one blade still and describe what happens when the other moves; the cut nevertheless depends on both. The image has endured because it does more than distribute credit between rival theories. It requires the economist to specify the demand and the supply together, to say what is being held fixed, and to allow enough time for the adjustment under discussion. A price is the meeting of those conditions, and a changed price needs an account of which conditions changed.

The supply blade records the terms on which additional output can be brought forward, and its meaning depends on the decision being studied. For a small producer who takes the market price as given, another unit adds that price to revenue. If the extra cost is less, producing it increases profit; if the extra cost is greater, it reduces profit. At a smooth interior maximum, with the appropriate curvature and the decision to operate already justified, marginal cost equals price. That familiar rule is a conditional account of competitive supply. A seller whose additional sales require a lower price must instead reckon with marginal revenue, including the effect of that lower price on the other units sold. And an industry's normal supply price over a longer period has to sustain the equipment, organization, and entry needed to keep production going. Marshall includes depreciation, interest, insurance, wages, and management in that reckoning. A momentary offer of a good already made and the price expected to maintain its manufacture answer different questions; the diagram becomes useful only when we know which one its supply curve answers.¹

The crossing of the blades identifies a quantity that buyers will take on terms sufficient to bring that quantity forward. Consider an illustrative market in which the demand price for a quantity q is 100 minus q, while the supply price is 20 plus q, with prices and quantities measured in fixed units. At forty units both prices are sixty. At thirty units buyers would pay seventy for the marginal unit, while the supply schedule asks fifty; at fifty units those figures are fifty and seventy. The crossing thus distinguishes an amount for which further expansion has support from one for which it does not. Now raise the demand schedule to 120 minus q, leaving the supply schedule unchanged. The new meeting is at fifty units and a price of seventy. Both price and quantity rise because the extra demand encounters a rising cost of supply. Neither the old price nor the new one is explained by reciting only the buyers' desire or only the sellers' expense. The result comes from putting two specified schedules into relation.²

Time alters what supply can do, and therefore alters the result of the same change in demand. The fish already on the slabs cannot be multiplied before the morning's sale is over; boats and crews can respond over a longer interval. If our illustrative market has exactly forty units available and all must be sold, the original demand schedule gives a price of sixty and the higher demand schedule a price of eighty. There is a price response with no immediate quantity response. At the other extreme, suppose a competitive industry can reproduce additional units at a constant normal supply price of twenty, with capacity and entry free to adjust. The two demand schedules then give quantities of eighty and one hundred at that same price. Demand still matters: it determines how much is produced. The contrast explains why Marshall generally gives greater weight to demand in very short periods and more attention to production costs as the period lengthens. It also marks the condition behind the strongest version of the long-period claim. A constant normal price requires the relevant constancy of supply costs; the mere passage of time does not make every industry's supply curve horizontal.³

Holding other things equal makes the two-blade experiment intelligible, while also drawing its boundary. Marshall's account temporarily fixes tastes, available substitutes, technical knowledge, and the broader conditions under which the market operates. These are the background against which a changed demand or supply schedule can be followed. A higher cloth price observed in the world, however, could accompany stronger demand, a dearer input, a disrupted trade route, or several changes together. The price alone cannot identify the shift. Nor must every response remain inside the market drawn on the page: an industry's expansion can alter the wages it pays, the prices of its materials, or the alternatives available to its buyers. Those feedbacks may require a larger account, and the later plate on equilibrium will make the interdependence explicit. Partial analysis earns its place by isolating a mechanism precisely enough to examine it. Its discipline consists in remembering which influences were held still, and restoring a neglected influence when it changes the answer.⁴

A crossing also requires an account of adjustment before it can be called a resting place to which the market will return. Marshall describes production increasing when demand price exceeds supply price and decreasing when it falls short. Under the ordering of schedules he specifies around a stable intersection, those movements lead back toward the crossing. He then complicates his own pendulum image: the forces act amid disturbances, and the point toward which the market tends may itself be moving. The equality of two numbers on a page does not establish how quickly producers can change output, how expectations will be formed, or whether delays will magnify a disturbance. A curve that turns, a threshold for entry, or an economy of scale can also require more care than the familiar single upward supply curve suggests. Nor does a competitive crossing, by itself, settle distribution or include costs imposed outside the transaction. Stability, efficiency, and justice are further questions with their own conditions. The scissors locate the relation that a particular account of demand and supply proposes; the economist must still establish the kind of equilibrium that relation represents.⁵

Marshall's scissors turn the dispute about the source of value into an inquiry that can be worked. State what buyers will take at different prices; state what sellers can supply on different terms; specify the period and the conditions held fixed; then examine their meeting and the adjustments around it. Cost and usefulness each do work within that account, and their apparent importance changes with the capacity to respond. The fish on the stall, the factory operating with its present equipment, and the industry able to build another factory present different supply problems even when the buyer wants the same additional thing. This is the instrument's lasting economy: it keeps both sides in view while making the particular comparison manageable. Behind the schedules lies the recurring decision of whether to do a little more. The next plate takes that decision out of the market diagram and states the method it shares with the rest of economic choice.