The Logic Behind a Progressive Tax

Introduction and disclaimer: Please note that I don't necessarily subscibe to this logic but I've recently had the logic behind the progressive tax explained to me.

The progressive tax is the hallmark of both the Democratic Party and the Green Party. It is the scourge of conservatives. Conservatives view the progressive tax system as patently unfair and punishing success. So, what is the logic behind the progressive tax system?

Those favoring a progressive system that the first $20,000 anyone makes is much more vital than the second million. So, you probably need all of the first $20,000 you make and so a progressive wouldn't want any of that taxed. You probably need most of the next $10,000 and so that's taxed vey slightly. Now, you probably need little of your second million and so that's taxed very heavy. So, the logic behind the progressive tax is entirely based on need.