The only way to understand the Court's Commerce Clause jurisprudence is by turning to chaos theory. Chaos theory tells us that if a butterfly flaps its wings in Hong Kong, it may cause a hurricane in Texas. "Fundamental to chaos theory is the phenomenon of sensitive dependence on initial conditions, commonly referred to as the Butterfly Effect." Leonardo Electronics Almanac.
If I sneeze in California, it may cause an earthquake in Missouri. Hence, Congress has the power to criminalize my intrastate sneezing because it may substantially affect interstate commerce. (After all, an earthquake can cause billions of dollars in damage.) Everyone has heard of the million dollar man. But had you heard of the billion dollar sneeze?
Although my tongue was firmly in cheek when I wrote that post, it's not far from off. In Raich, the Court held that commerce is anything that Congress has a rational basis for concluding is commerce. And, according to Justice Breyer, here is what counts as a rational basis:
[W]e can imagine a reason, but if you look at it realistically, you know there's no good reason.... I mean, if you put your mind to it, you can make one up, which is sort of the test for rational basis in the economic context. But as soon as we become realistic, there isn't much of a reason.
Pretty close, 'eh?
UPDATE: Darn, darn, darn! Someone else beat me to the butterfly effect angle. Via Sandefur:
Perhaps you know this, but the Sixth Circuit Court of Appeals adopted your chaos theory reference in United States v. Wang, 222 F.3d 234, 239 (6th Cir. 2000):
[W]hen the United States argued that gun possession in school zones would, in the aggregate, result in violent crime which would result in costs which would affect the national economy through the mechanism of insurance, the Court responded: "To uphold the Government's contentions here, we would have to pile inference upon inference in a manner that would bid fair to convert congressional authority under the Commerce Clause to a general police power of the sort retained by the States." Lopez, 514 U.S. at 567, 115 S.Ct. 1624.... Just this sort of "butterfly effect" theory of causation would be required to find liability in the great majority of Hobbs Act cases in which the victim is a private citizen. See James Gleick, Chaos: Making a New Science 8 (1987) (discussing the parable of the flapping of a butterfly's wings that creates a minor air current in China, that adds to the accumulative effect in global wind systems, that ends with a hurricane in the Caribbean).
FN1. This might also be viewed as the "dog, dog bite pig" theory of causation. See The Little Old Woman and Her Pig, in The Tall Book of Nursery Tales 92 (1972). The little old woman had been stymied in her attempt to get home because her recalcitrant pig refused to cross a stile. So the old woman gave water to a haymaker for a wisp of hay to give to a cow for some milk to induce a cat to begin to kill a rat that began to gnaw a rope that began to hang a butcher who began to kill an ox who began to drink some water that began to quench a fire that began to burn a stick that began to beat the dog who began to bite the pig who jumped over the stile in a fright. Id. at 97. While this sequence of events got the little old woman home that night, such a causal chain will not suffice to put Mr. Wang in federal court.