📜 Where this idea comes from

Venn diagrams

In 1880 the English logician John Venn drew overlapping circles to check arguments, in a paper for the Philosophical Magazine. Leonhard Euler had used circles for logic more than a hundred years earlier, in letters he wrote to a German princess.

One thing to hold: Two or three overlapping circles, with a region shaded out when nothing is in it.

Why it matters: Shading turns words like “no” and “all” into a picture, so you can check a claim by looking.

Sources: John Venn, “On the Diagrammatic and Mechanical Representation of Propositions and Reasonings”, Philosophical Magazine (1880) · Leonhard Euler, Letters to a German Princess (1768)

The Game of Logic

Lewis Carroll — the mathematician Charles Dodgson, who wrote Alice’s Adventures in Wonderland — published The Game of Logic in 1887. It came in an envelope with a board and nine counters, four red and five grey.

One thing to hold: A square board marked with circles, and counters to say “some are” or “none are”.

Why it matters: Putting counters on the board turns two statements into a picture, and the conclusion can be read straight off it.

Sources: Lewis Carroll, The Game of Logic (Macmillan, 1887) · Wikipedia, “The Game of Logic”