host of other issues (e.g., tax planning, compliance, and administrative costs, not to mention other types of information that are relevant to distribution). Now take the carbon tax. We know what we don't like: carbon's effect on global warming. Happily (or not), all carbon atoms are relevantly the same. So we know what we are trying to tax and why, and this really helps in designing a carbon tax -- leaving aside the issue of global cooperation, since the scope of the public good (moderating climate change) is global. Now let's turn to the FTT. What are we trying to tax and why? Even if on balance adopting the tax would have good consequences relative to not adopting it, it's not clear what the "thing" we're after is. And this makes it really hard, for example, to say how derivatives should be treated under the FTT. Suppose that what we are after is excessive effort to seek trading gains. Even with just old-fashioned stock trading, it's not entirely clear how good a proxy gross trading volume is for the thing we have in mind. For example, do we want to tax repos that involve financial instruments? These are transactions in which I "sell" you a financial asset for $X, but we agree that I will buy it back at a given time for $X + $Y, where Y is essentially an interest return. This is really just a way of doing a secured loan, which principally wouldn't be taxed if