Moral Algebra
Background
In the summer of 1772 the English scientist and minister Joseph Priestley — soon to be famous for discovering oxygen — had a big decision to make. The Earl of Shelburne had offered him a well-paid post as his librarian and companion, with time for his experiments. Taking it meant giving up his congregation in Leeds and uprooting his family.
He asked his friend Benjamin Franklin, then living in London, what to do. Franklin’s reply declines to answer the question and gives him something better: a method. Divide a sheet of paper, list the reasons on each side over a few days, weigh them, cross out the ones that cancel, and see where the balance lies.
Priestley took the job. The method is still the one most of us reach for when we’re stuck.
The letter
London,
September 19, 1772
Dear Sir,
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 take 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
Franklin’s spelling and capitals, from The Papers of Benjamin Franklin (read it at Founders Online). More Franklin in the Benjamin Franklin Quote Library and his profile. If you’re weighing a big decision, the Values Exercise is a good first column.