Franklin uses the language of finance to describe the value of learning. An investment normally involves giving up resources in the present in the expectation of a future return. In the same way, acquiring knowledge requires time, effort or money, but can continue to provide benefits through better judgement, greater competence and expanded opportunities.

The reference to the “best interest” is metaphorical rather than a claim that education always produces the highest financial return. Knowledge may provide benefits that are intellectual, practical or personal as well as economic. Its value also depends on how effectively it is understood and applied. The broader point is that learning can remain useful long after the original effort has been made, making education a form of investment whose returns may accumulate throughout a person’s life.

Full quotation

“If a man empties his purse into his head, no man can take it away from him. An investment in knowledge always pays the best interest.”