In The General Theory of Employment, Interest and Money, Chapter 2: The postulates of the classical economics, John Maynard Keynes says:
It would follow from this that there are only four possible means of increasing employment:
(a) An improvement in organisation or in foresight which diminishes 'frictional' unemployment;
(b) a decrease in the marginal disutility of labour, as expressed by the real wage for which additional labour is available, so as to diminish 'voluntary' unemployment;
(c) an increase in the marginal physical productivity of labour in the wage-goods industries (to use Professor Pigou's convenient term for goods upon the price of which the utility of the money-wage depends);
or (d) an increase in the price of non-wage-goods compared with the price of wage-goods, associated with a shift in the expenditure of non-wage-earners from wage-goods to non-wage-goods.
I understood how (a) and (b) can increasing employment but I don't sure if I have understood (c) and I completely sure I don't understood (d).
I think (c) means that If in the wage-goods industries are more productive then wage-goods will be cheaper and then disutility of labour decrease because of with lower wages you can buy the same, so (c) is follow by (b). Is that what Keynes want to say?
Also, I don`t understood if "marginal physical productivity" means something different than "marginal productivity".
And, as I said, I completely sure I don't understood (d). How (d) increasing employment?