While we’re certainly not close to the end of science – so many profound mysteries remain – we should be prepared to work harder for what we learn next. All the low-hanging facts have been found.Lehrer writes primarily about medicine and neuroscience, and certainly we're unlikely to discover a new component of a cell or even of the brain. But that's a very short-sighted point of view.
In particular, we don't even know the facts about the simpliest question you could ask: what is the world made of? We have essentially no idea. Physicists have determined that about 70% of it is made up of "dark energy" and about 25% of it "dark matter," but they have absolutely no idea what these quantities are made of. None (especially dark energy).
You simply can't say that "all the low-hanging facts have been found" when you have no idea what 95% of the universe is made of.