folk Freud

When a suckling babe bites the nipple, the flow of milk stops.  And this is the ideal in punishment: there is correspondence and proportion. 

The correspondence is to what the babe wants: milk is wanted, and it is the supply of milk that is interrupted.  The correspondence is not to what the mother suffered.

The proportion is to motivation.  A young babe had little or no motive to bite and the interruption in suckling is for only a few seconds.  An older baby, perhaps, is motivated by temper, and the interruption is longer.

This ideal applies throughout human punishment: when possible, the punishment should correspond to the desire connected to the crime and should be proportioned to counter the bad motivation, with a margin sufficient to public safety (this is why fines and imprisonment should, in my view, be indexed to wealth as well as sized to harm).  Practically, this is often a sharp, sudden, sizable interruption to the flow of what is desired: power over others, money, pleasure, and so forth.

Now, the ideal may not be possible.  And when it isn't, aim for correspondence in time, that is, immediacy, and for limited cost, that is, avoiding excess trouble, resistance, and risk of resentful learning of the wrong lesson.  It is in this light that corporal punishment, never ideal, may be the least bad option.