What is the meaning of this quote by Karl Marx? "Sell a man a fish, he eats for a day. Teach a man how to fish, you ruin a wonderful business opportunity."
This quote is a play on the adage, "Give a man a fish, and he will eat for a day. Teach a man to fish, and he will eat for a lifetime." Marx's alteration of the second half is a commentary on the the ways in which business can exploit personal needs and capabilities.
One of the basics of business is the relationship between supply and demand. In the case of this quote, the demand is for fish. If a man does not know how to fish (or otherwise acquire food,) he must buy fish from someone else. The supply is coming from someone who does know how to fish and can profit by selling their fish to others.
The man who knows how to fish and sells what he catches can be considered to be exploiting the needs of the man who does not know how to fish. Many businesses supply goods and services that others cannot perform for themselves. While this is a reasonable course of action and business model, as not everyone can know how to do everything on earth, the moral or ethical debate has to do with being able to provide for oneself.
While selling or giving a man a fish does meet his immediate needs, it does not help him to provide for himself in the long-term. The man who knows how to fish could help the first man by teaching him, but this would eliminate the demand for fish and ruin his opportunity for profit.