Q. In Kohelet it says, “Cast your bread upon the waters”. How did this idea originate?
In Kohelet’s time it is not likely to have referred to the financial markets since such institutions did not then exist. More probably it has a moral connotation: do a mitzvah today and it will bring you benefits in the future.
True, it seems to say that one should perform good deeds with a thought of reward, even though Pir’kei Avot tells (1:3) us to “serve the Master” without ulterior motive, doing good deeds for their own sake. But maybe Kohelet is speaking as an observer rather than an ethicist and he has noticed that something you do today tends to have an effect days, years and sometimes decades, later.
Rashi takes the word “bread” rather literally in this connection and reminds us that Jethro invited Moses – whom he thought to be an Egyptian stranger – for a meal (literally, “to eat bread”: Ex. 2:20). Jethro did a good deed, gained a son-in-law and changed the whole course of history.
Why the verse says “upon the waters” is that the waters flow and move and a good deed goes with them like a person on a ship whose exact date of return may be uncertain.