To Joseph Priestley
ALS (letterbook draft) and copy: Library of Congress
In July, 1772, Lord Shelburne asked Priestley to be his librarian. The offer, although both financially and personally appealing, put the scientist into a quandary. On the one hand, the Earl was as attractive as the salary: “for ability and integrity together, the very first character in this kingdom,” a friend of dissenters and one who would be an influential patron. On the other hand, Priestley’s life in Leeds was so comfortable and productive that he had not contemplated moving, except perhaps to America.5 He wrestled long with the decision, and did not accept until December.6 Meanwhile he consulted Franklin, who suggested a solution to the problem by way of “prudential Algebra.”7
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
5. Schofield, Scientific Autobiog., pp. 105–6. Priestley, it should be added, was one of the few people in the kingdom who took this view of “the Jesuit of Berkeley Square.”
6. Ibid., pp. 105–6, 108, 111–12; John T. Rutt, Life and Correspondence of Joseph Priestley … (2 vols., London, 1831–32), I, 187–8.
7. BF later urged the same system on Jonathan Williams, Jr.: Smyth, Writings, VII, 281–2.