Cantor's Theorem states that the cardinality of a set A is strictly less than the cardinality of its power set P(A), for every set A. In other words, there is no bijection between A and P(A).
The proof of Cantor's Theorem relies on a diagonalization argument. Suppose there is a bijection f between A and P(A). We can use f to construct a subset B of A that is not in the image of f.
To do this, we define B as follows: for each element x in A, if x is not in the set f(x), then we add x to B. In other words, B contains all elements of A that are not in their corresponding set in P(A) under f.
Now, we show that B is not in the image of f. Suppose that there exists some element y in A such that f(y) = B. Then, we have two cases: either y is in B or y is not in B.
If y is in B, then y is not in f(y), since y was added to B precisely because it is not in its corresponding set in P(A) under f. But this contradicts the assumption that f(y) = B.
If y is not in B, then y is in f(y), since y is not in B precisely because it is in its corresponding set in P(A) under f. But this also contradicts the assumption that f(y) = B.
Therefore, we have shown that B is not in the image of f, which contradicts the assumption that f is a bijection between A and P(A). Thus, there can be no such bijection, and Cantor's Theorem follows.
The consequence of Cantor's Theorem is that there are different sizes of infinity, which has profound implications for mathematics and philosophy. It shows that there are sets that are "larger" than others, and that there is no "largest" infinity. This has led to the development of set theory as a foundational branch of mathematics, and has influenced debates about the nature of infinity in philosophy.