Earlier, when value was treated simply as labor and was not given distinct social characteristics, value was equated with labor on one hand, and was separated from exchange value by an abyss on the other hand. In the concept of value economists frequently duplicated the same labor. From this concept of value they could not move to the concept of exchange value. Now when we consider value in terms of content and form, we relate value with the concept which precedes it, abstract labor (and in the last analysis with the material process of production), the content. On the other hand, through the form of value we have already connected value with the concept which follows it, exchange value. In fact, once we have determined that value does not represent labor in general, but labor which has the « form of exchangeability » of a product, then we must pass from value directly to exchange value. In this way, the concept of value is seen to be inseparable from the concept of labor on one hand, and from the concept of exchange value on the other.
Isaak Rubin « Content and form of value » Essays on Marx’s Theory of Value  trad. Milos Samardfija Fredy Perlman Black Rose Books 1973 p. 122