I think he means you determine if "the market is at a low point" by comparing to the recent past. For instance, today, I can tell the market is at a high point

I think he means you determine if "the market is at a low point" by comparing to the recent past. For instance, today, I can tell the market is at a high point. It might be a good time to rebalance from stocks to bonds today if that's something I'm looking to do. That the market will rise again has held in the past in the U.S., so it's not so unreasonable to suspect it will in the future.