I’ve created the following two sets:

A = {0, 1, 5, 9}

B = {sugar, spice, snips, snails}

Which one is larger?

In terms of the elements it contains, they both have 4 elements, so they are the same size.

The number of elements in a set is called the *cardinality* of that set, and it’s written so: |A| = |B| = 4

But how could you compare them if you couldn’t count?

What you can do is take one element from A, and one from B, and put them aside. If you run out of elements at the same time, then they have the same cardinality. This is known as **correspondence**, a one-to-one comparison of elements in each set.

Think of very large finite sets, like all of the people of the world. You could potentially assign each of them a number, and there would be both one number per person and one person per number. In effect, there would be a correspondence between the two.

But what about numbers themselves? Are they countable?

Continued tomorrow. . .

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