The future can be found in the past

If prices are based on investor expectations, then knowing what a security should sell for (i.e., fundamental analysis) becomes less important than knowing what other investors expect it to sell for. That's not to say that knowing what a security should sell for isn't important--it is. But there is usually a fairly strong consensus of a stock's future earnings that the average investor cannot disprove.
"I believe the future is only the past again, entered through another gate."- Sir Arthur Wing Pinero, 1893
Technical analysis is the process of analyzing a security's historical prices in an effort to determine probable future prices. This is done by comparing current price action (i.e., current expectations) with comparable historical price action to predict a reasonable outcome. The devout technician might define this process as the fact that history repeats itself while others would suffice to say that we should learn from the past.