The annual produce of the land and labour of any nation can be increased in its value by no other means, but by increasing either the number of its productive labourers, or the productive powers of those labourers who had before been employed.
Adam SmithMen of the most robust make, observe that in looking upon sore eyes they often feel a very sensible soreness in their own, which proceeds from the same reason; that organ being in the strongest man more delicate, than any other part of the body is in the weakest.
Adam Smith