You bring up a great point. To classify something as a trend or not a trend, we have to use this artificial black box to supply ourselves with examples of what's a trend and what isn't. The nice thing, IMO (and this is something I admittedly gloss over at the very end) is that doing prediction/forecasting with this method is almost the same as doing classification, even though you don't have any labeled examples when doing prediction. To do classification, we compare current activity to past examples of activity, and decide if it looks like the positive examples or the negative examples. For prediction, we compare current activity to past activity, and see how similar-looking past activity continued to evolve over time.