Argument: Progressive taxation applies a compassionate philosophy to poor
"In support of progressive taxation". The visible hand in economics. 12 Nov. 2008 - People value the idea that poor people are being helped out, however any individual income they provide may not help people very much. Furthermore, if we are in a situation where everyone else is giving income to the poor, this person does not need to and so won’t - as a result there is a prisoner’s dilemma. People want society (including them) to provide for the poor, but have an incentive to pike out themselves, leading to a worse outcome for everyone. In this case a progressive tax can help to circumvent the prisoners dilemma.
Robert J. Shapiro. "Flat Wrong: New Tax Schemes Can't Top Old Progressive Truths". Washington Post. 24 Mar. 1996 - "The point of progressive taxation is not to penalize those who succeed, but to protect those who have not."