In an often cited letter to the British scientist Joseph Priestley, Franklin
explained how to decide which of two options to take, based on uncertain cues (which he calls 'reasons'):
"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 those 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 alternatively 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 of 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 endeavor 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 . . . and thus proceeding I find at length where the balance lies;
and if, after a day or two of further consideration, nothing new that is of importance occurs on either side, I come to a
determination accordingly.
And, though 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 liable to make a rash step, and in fact I have found great advantage from this kind of equation."
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