However much we would like advertising to be a science - because life would be simpler that way - the fact is that it is not. It is a subtle, ever-changing art, defying formularization, flowering on freshness and withering on imitation; where what was effective one day, for that very reason, will not be effective the next, because it has lost the maximum impact of originality.

As quoted in Bill Bernbach said... (1989), DDB Needham Worldwide.