According to Cantor, two sets \(A\) and \(B\) have the same size, or cardinality, if they are bijectable, i.e., the elements of \(A\) can be put in a one-to-one correspondence with the elements of \(B\). Thus, the set \(\mathbb{N}\) of natural numbers and the set \(\mathbb{R}\) of real numbers have different cardinalities. In 1878 Cantor formulated the famous Continuum Hypothesis (CH), which asserts that every infinite set of real numbers is either countable, i.e., it has the same cardinality as \(\mathbb{N}\), or has the same cardinality as \(\mathbb{R}\). In other words, there are only two possible sizes of infinite sets of real numbers. The CH is the most famous problem of set theory. Cantor himself devoted much effort to it, and so did many other leading mathematicians of the first half of the twentieth century, such as Hilbert, who listed the CH as the first problem in his celebrated list of 23 unsolved mathematical problems presented in 1900 at the Second International Congress of Mathematicians, in Paris. The attempts to prove the CH led to major discoveries in set theory, such as the theory of constructible sets, and the forcing technique, which showed that the CH can neither be proved nor disproved from the usual axioms of set theory. To this day, the CH remains open.