"The system needs fewer degrees of freedom.  If there's three goods you can trade, there should only be two trading ratios.  Once you know apples to bananas and bananas to cherries, that gives you apples to cherries.  So if you make up a separate number there that isn't derived from the first two, that's like having two different trading ratios at two different times, or different trading ratios depending on the order you trade things in."

"The constraint system needs fewer degrees of freedom.  If there's three goods you can trade, there should only be two trading ratios.  Once you know apples to bananas and bananas to cherries, that gives you apples to cherries.  So if you make up a separate number there that isn't derived from the first two, that's like having two different trading ratios at two different times, or different trading ratios depending on the order you trade things in."

"The constraint-system needs fewer degrees-of-freedom.  If there's three goods you can trade, there should only be two trading ratios.  Once you know apples to bananas and bananas to cherries, that gives you apples to cherries.  So if you make up a separate number there that isn't derived from the first two, that's like having two different trading ratios at two different times, or different trading ratios depending on the order you trade things in."

"The constraint-system needs fewer degrees-of-freedom - one fewer than the number of goods being traded.  If there's three goods you can trade, there should only be two trading ratios.  Once you know apples to bananas and bananas to cherries, that gives you apples to cherries.  So if you make up a separate number there that isn't derived from the first two, that's like having two different trading ratios at two different times, or different trading ratios depending on the order you trade things in."

"The constraint-system needs fewer degrees-of-freedom - one fewer than the number of goods being traded.  If there's three goods you can trade, there should only be two trading ratios.  Once you know apples to bananas and bananas to cherries, that gives you apples to cherries.  So if you make up a separate number there that isn't derived from the first two, that's like having two different trading ratios at two different times, or different trading ratios depending on the order you trade things in.  The constraint-system starts with zero degrees-of-freedom for one fruit, and then any time you add a new fruit, you add one degree-of-freedom for a trading ratio between that fruit and any other fruit already in the constraint-system."