[wombat23]

Well it kinda is.

Empirically, you could have bought a share of the SPX at any point in time, and sold it with profit later. The real problem is what happened in the time between, and whether you were able to hold on.

[dmos62]

You can buy shares in an index?

[danuker]

You can buy shares in an ETF (exchange-traded fund) that tracks an index (buys and sells to attempt to keep exposure the same as the index).