That and Which (2)

That and Which (2)

In English grammar, 'which' describes while 'that' defines. In some cases it is impossible to decide which of these two pronouns is correct. In the following, however, it is quite clear that John Maynard Keynes is wrong.

"Since we are here concerned in determining what sum will be spent on consumption when employment is at a given level, we should, strictly speaking, consider the function which [that] relates the former quantity (C) to the latter (N)." ( "The General Theory of Employment, Interest and Money", chapter 8, [1])

 I conclude that JMK has a tendency to confuse these two relative pronouns. In which case we can not trust him when he writes ( my emphasis): 

"....; whereas they do not resist reductions of real wages, which are associated with increases in aggregate employment and leave relative money-wages unchanged, unless the reduction proceeds so far as to threaten a reduction of the real wage below the marginal disutility of the existing volume of employment." -- (in JTEIM chapter 2)

"Which" would be correct if all reductions of real wages are of the sort described, but are they? 

References:

1.  John Maynard Keynes (1936), "The General Theory of Employment, Interest and Money"