The financial sector has imposed huge externalities on the rest of society. America's financial industry polluted the world with toxic mortgages, and, in line with the well established "polluter pays" principle, taxes should be imposed on it. Besides, well-designed taxes on the financial sector might help alleviate problems caused by excessive leverage and banks that are too big to fail. Taxes on speculative activity might encourage banks to focus greater attention on performing their key societal role of providing credit.

Joseph Stiglitz (The Dangers of Deficit Reduction)

I have many favorite books and quoters.