I have been thinking about the idea of 'regression to the mean' a bit recently. It's a statistical term to describe the statistical tendency for 'outliers' or extremes to move toward an average value.
We see this play out in the prices of stocks/equities. Just don't pretend that you can play the game better than the market (unless you're Warren Buffet!)
Things go up, things go down but the duration of each up or down phase is not something we can predict very well. Nor can we predict turnarounds very well. But one thing seems true...we'll see ups and downs over time.
When it comes to depression - I think we can fall into the trap of thinking things are going to be up forever when things are going up...and when things regress to the mean (by going down) we think everything is all downhill...
...but things do change...eventually...we just don't know when.
So while we cannot predict things in the future, we can prepare.
If you know that things regress to the mean, perhaps we won't get so caught up in the 'what ifs' and 'should haves' and 'could have's when we're on the up, or on the down...