Department I's efforts to accumulate by selling its additional product to Department II have met with an unlooked-for result: a deficit for the capitalists of Department II serious enough to prevent even simple reproduction on the old scale. Having got to this crucial point, Marx seeks to lay bare the root of the problem by a careful and detailed exposition: 'Let us now take a closer look at the accumulation in Department II. The first difficulty with reference to II_c_, that is to say the conversion of an element of the commodity-capital of II into the natural form of constant capital of II, concerns simple reproduction. Let us take the formula previously used. (_1,000v + 1,000s_) I are exchanged for 2,000 II_c_. Now, if one half of the surplus-product of I, or 500_s_, is reincorporated in Department I as constant capital, then this portion, being detained in Department I, cannot take the place of any portion of II_c_. Instead of being converted into articles of consumption, it is made to serve as an additional means of production in Department I itself.... It cannot perform this function simultaneously in I and II. The capitalist cannot spend the value of his surplus-product for articles of consumption, and at the same time consume the surplus-product itself productively, by incorporating it in his productive capital. Instead of 2,000 I(_v + s_), only 1,500 are exchangeable for 2,000 II_c_, namely _1,000v + 500s_ of I. But 500 I_c_ cannot, be reconverted from the form of commodities into productive constant capital of II.'[124]