Leibniz (who invented calculus as you learn it in school) invented a system where along each edge of each card was a row of squares with either a hole or a notch open to the edge. Each square corresponded to a category or property. To select all the cards that conformed to either of two categories, you stuck rods through the stack at the corresponding squares, and lifted: OR. For AND, you would use one rod and lift, and then again in the cards lifted, at the second position. For NOT, you use the cards left behind. So you could apply any arbitrary boolean expression to as many properties as you had squares on the edges. He meant to categorize everything in the world that way.

That method was used for searching in a data set for centuries, until punch-card sorting machines, and then computers. Index cards used to be essential for organizing anything.

I went from one end of Indonesia to the other looking for index cards, to make flash cards for vocabulary. They could not be had for love or money. But I finally found "visiting cards".