Moreover, within this outermost Limit of Value, which is made by the comparative onerousness of the respective "efforts," there is a second limitation of a similar kind to be found specially in the element which we have called "estimates." The estimate of each exchanger is based at once on his own effort about to be rendered and on his desire for the return service offered: the element of effort in the case of both being considered for the time as fixed, Value will vary according to the varying desire of each for the return service of the other, affecting of course the "estimate" of each, and furnishing also a secondary Limit of Value. To pursue the same illustration, suppose I regard the effort required to bring the water myself as 10; that there are several persons, who would be glad to do that service for me at a return service which I consider as 8; that there are two persons, who are willing to do it for something which I estimate at 6; and that there is only one person, who will do it for a return service which I regard as 5. It is evident, that the extreme limits of that service to me are 10 and 5. Higher than 10 it cannot go, lower than 5 it cannot sink. But why have I before me three possible classes of renderers? Because the persons in each class, while estimating their own efforts alike in the proposed rendering to me, have varying "desires" as towards a possible rendering from me to them, and consequently put differing "estimates" upon the possible transactions. The man who will bring the water for 5 has for some reason (no matter what) a stronger desire for the return than anybody else, and I should of course employ him so long as he would serve me on those terms; if he decline the exchange, I fall back on one of the two persons in the class above him, and Value rises now from 5 to 6, and will be steadier there than it was before; if each of these in turn should give out, I should fall back upon the larger class ready to serve me at 8, and Value would be very steady at that rate, because there are numerous competitors; and by no possibility could it rise above 10. Between 10 and 5 the value may fluctuate, but it cannot overpass these Limits in either direction under existing circumstances. Therefore we may conclude, that the _maximum_ Value of any Service in exchange will be struck at the point where the recipient will prefer to serve himself, or go without the satisfaction, rather than make the exchange; and the _minimum_ Value of any Service in exchange is struck at the point below which the recipient cannot get himself served even by him who most highly estimates the return service offered. (4) We come now to the last and most important Inquiry in this initial chapter, namely this, _Can there be, and is there, a strict Science of Buying and Selling? Is there a Science by itself, clear and certain, that covers and controls Valuables?_