Memo to Students: Ben Franklin’s Prudential Algebra

Benjamin Franklin wrote this to a friend in 1772:

To Joseph Priestley

Dear Sir, London Sept. 19. 1772 In the Affair of so much Importance to you, wherein you ask my Advice, I cannot for want of sufficient Premises, advise you what to determine, but if you please I will tell you how. When these difficult Cases occur, they are difficult chiefly because while we have them under Consideration all the Reasons pro and con are not present to the Mind at the same time; but sometimes one Set present themselves, and at other times another, the first being out of Sight. Hence the various Purposes or Inclinations that alternately prevail, and the Uncertainty that perplexes us. To get over this, my Way is, to divide half a Sheet of Paper by a Line into two Columns, writing over the one Pro, and over the other Con. Then during three or four Days Consideration I put down under the different Heads short Hints of the different Motives that at different Times occur to me for or against the Measure. When I have thus got them all together in one View, I endeavour to estimate their respective Weights; and where I find two, one on each side, that seem equal, I strike them both out: If I find a Reason pro equal to some two Reasons con, I strike out the three. If I judge some two Reasons con equal to some three Reasons pro, I strike out the five; and thus proceeding I find at length where the Ballance lies; and if after a Day or two of farther Consideration nothing new that is of Importance occurs on either side, I come to a Determination accordingly. And tho’ the Weight of Reasons cannot be taken with the Precision of Algebraic Quantities, yet when each is thus considered separately and comparatively, and the whole lies before me, I think I can judge better, and am less likely to make a rash Step; and in fact I have found great Advantage from this kind of Equation, in what may be called Moral or Prudential Algebra. Wishing sincerely that you may determine for the best, I am ever, my dear Friend, Yours most affectionately, B Franklin


Note the underlying wisdom in the Franklin system of decision making. By making a list of things that would go in favor of, as well as, a list of things that would go against, a potential decision, he prevented his mind from jumping to conclusions (first conclusion bias as a subset of availability bias). This procedure also ensured that Franklin did not over-weigh any particular factor which could mis-influence his decision (availability bias). Further, his insistence on not making sudden decisions without reflecting over them for a few days ensured that he “preserved optionality” allowing new information to come in which could change the odds (Robert Rubin’s idea of preserving optionality is valid here). Finally, his roughly-right system of dealing with trade-offs works way better, in my view, than the “optimization systems” you see in the modern world.

Keynes had it right when he said, “its better to be roughly right than to be precisely wrong.”

Btw, I love doing “Prudential Algebra” on my mind maps

One thought on “Memo to Students: Ben Franklin’s Prudential Algebra

  1. I used this method ( without knowing the name of it or full attribution) to decide amongst science and commerce

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