Double taxation

Imagine you are a doctor who has an extra $10k to spend after a year of budgeting. You have narrowed down your purchase decision to two options: a pair of new jet-skis, or an investment in a local restaurant. From the jet-skis you get a return of glorious fun with perhaps a special someone, and from the restaurant investment you get a return of equity and the capital gains that comes from dividend payouts. The returns from the jet-ski option are not taxed by the government besides sales tax and licensing (you can't tax fun!), but the returns from investing the restaurant - I suppose this is responsible behavior - well, those are subject to a whole bunch of taxes.

This has always been my understanding of the argument against double taxation, that you have earned the money once by seeing patients and were taxed then, and now you will be taxed a second time if you take the responsible option of investing your money. How unfair! Let's cut capital gains tax!

Actually, it's sort of the same idea with labor.

If I have 40 hours a week to toss around, I can lay in bed all day reading blogs (patently irresponsible) or I can get a job. I've taken my time and have done a responsible thing with it and now I get taxed for it, while the government has not found a way to tax my blog reading. Unfair!

But wait, that's not double taxation. So is double taxation actually important? Why is taxing the money I make working so much better than taxing the money I make investing? Perhaps it is that time flows to use freely, while the money to invest had to be earned.

Also, I've got something on equivalence of consumption and income taxes and Say's law, but I'll post later when I'm not exhausted.