I'm looking for an SF short story in which the protagonist has a vision of the future in which all humans end up living in tanks filled with a plant-like or algae-like symbiont. The people in the tanks have the subjective experience of living in a simulated paradise. The story is quite old, possibly from the 1920s or 1930s.
Possibly The Higher Things by J. R. Pierce, though that is from 1970 not the 1920s or '30s. The protagonist of the story, Jimmy, describes a meeting with Haskel van Manderpootz who is a stereotypical mad scientist. Manderpootz builds a machine that allows him to see the future, and he tells Jimmy:
“I didn’t know what to do, Jimmy. I, the great van Manderpootz. I was so hurt, and so disappointed to doubt the future which I had helped to bring about, or which I would help to bring about if I continued with my biology and psychology. But I can face the truth. Van Manderpootz is no coward. So I went back to the machine, and I explored time back from that remote future in which I had been. And do you know what I found, Jimmy? Symbiosis! I found men preparing closed tanks, filled with all the elements needful to life, and with glass tops faced to the sun. And I saw them lowering unconscious human bodies into those tanks, into a mess of what looked like slime and must have been alive. And I knew what had happened. Man had found a way to live without effort, in the partnership with some simple organism, his body cradled and fed and kept young through the ages. And he had found telepathy as well. Finally, the very last man to walk the face of the earth had lowered himself into a tank and closed the lid. And men had lived in a world of dreams, thinking their thoughts together. They had forgotten their bodies, which lay on the face of the earth.