As the market slides, I have - optimistically - begun a systematic program of buying index fund shares. Stocks are on sale, right? One cannot guess how low the markets will dip, so the solution is to dollar-cost-average: spread the buying out over many months and thus buy an average-of-the-low rather than a single purchase at the abyss. Obviously this strategy depends for success on a rebound in stock prices.
What basis is there for long-term optimism? I cling to these factors: Trump will be gone sooner or later, and a degree of predictability will return to at least the US-centric realm of world affairs. Automation is progressing apace and will not stop for recession or wars - it will decrease labor costs in the long-term and thus enhance profitability. The underlying data on the US economy are still strong enough to support economic growth, according to government figures posted recently (December 2018).
I acknowledge the causes for pessimism: damage to the world order from Trump's thrashing about on the world stage, increasing risk of nuclear war on various fronts (really bad for business), potentially more rapid environmental effects from climate change than have been predicted (leading to loss of critical infrastructure like ports, refineries, and agricultural lands; and displacement of large populations).
Timing is everything. In the good old days, stock market dips sorted out after a couple of years. Some of the downside risks we face could exert significant drags on the world economy for longer spans, but their advent is also uncertain and may not arise in the current economic cycle at all. Then there is a sort of fatalism. If there IS nuclear war, who CARES what happened to my stock portfolio? Some kinds of disaster can be left off the financial-planning table because they are simply not the sorts of things one can plan around.
Anyway, this seems like a good time to shut up and let the community straighten me out :D