If I spend 1 hour producing fish, and you spend 1 hour producing linen, then, provided we both have a need for these items, we can exchange them rather than me spending 0.5 hours producing fish and 0.5 hours producing linen, and you doing the same, especially as by specialising, we are likely to produce both items more efficiently. Likewise, provided we both continue to have a need for these items, if I spend 2 hours producing fish, and you spend 2 hours producing linen, and output of both doubles, we can continue to trade.
It is as though both of us have simply doubled our consumption, and the same would be true if the output doubled due to a rise in productivity. In other words, it would be as though I now consumed twice as many fish as before, and you consumed twice as much linen. I have merely traded a part of my consumption of fish for consumption of some of your linen, and vice versa.
Why would we do this? One reason has already been mentioned, which is that by each of us specialising, we can produce more efficiently. But, we could both specialise in this way and yet not trade. The other reason to trade, therefore, is that I only have a demand for a certain quantity of fish, just as you only have a demand for a certain quantity of linen. Beyond those points, fish has no use value for me, just as linen has no use value for you.
This exchange can only occur in relation to those commodities which are for consumption, and, therefore, form part of revenue. If you produce looms, these are not a part of consumption, and so I cannot take part of my consumption in the form of a loom, because I cannot consume a loom.
My consumption is set within definable limits. As a worker, I must consume that minimum required for my reproduction. As a capitalist, I also need a minimum of consumption for my reproduction, and of the rest of my profit, I will be driven to accumulate a proportion to remain competitive, leaving another portion to consume as luxuries.
Consequently, if I exchange my fish for your loom, I must exchange the loom for linen, because the reproduction of my labour-power requires the minimum of consumption, of which the linen is part. Once again, in terms of Robinson Crusoe, if he needs to eat 4 fish per day, for the required calories, he can spend less time catching fish, provided he uses his time to catch rabbits, or collect fruits. But, he cannot exchange this time to produce fishing nets, because although they may help him catch more fish in future, they do not meet his immediate need for consumption, without which he will die. The same applies when he is joined by Friday. Their first requirement is to produce enough of what they need to consume.
“Secondly, it is just as clear: that only regarding this part of the exchange of products it is true that the producer’s supply is equal to the demand for other products which he wishes to consume. Here in fact it is only a question of a simple exchange of commodities. Instead of producing his means of subsistence himself, he produces the means of subsistence for another, who produces his.” (p 233)
However, its not the fact that both my fish and your linen constitute revenue, i.e. they represent the new value that our labour has created, which determines this exchange, but the fact that they are both consumable articles. In other words, we both require a certain quantity of consumable items. That part of my consumption needs, which I do not now produce for myself, I obtain from you, and vice versa.