Principle of Finite Choice

Theorem
Let $I$ be a non-empty finite indexing set.

Let $\family {S_i}_{i \mathop \in I}$ be an $I$-indexed family of non-empty sets.

Then there exists an $I$-indexed family $\family {x_i}_{i \mathop \in I}$ such that:
 * $\forall i \in I: x_i \in S_i$

That is, there exists a mapping:
 * $\ds f: I \to \bigcup_{i \mathop \in I} S_i$

such that:
 * $\forall i \in I: \map f i \in S_i$

Proof
We use the Principle of Mathematical Induction on the cardinality of $I$.

Basis for the Induction
Let $\card I = 1$.

Let $j \in I$.

Then $I = \set j$.

By the definition of a non-empty set, there exists an $x \in S_j$.

Hence, there exists a mapping $f: I \to S_j$ such that $\map f j = x$.

Since $\forall i \in I: i = j$, the result is indeed seen to hold for $\card I = 1$.

This is the basis for the induction

Induction Hypothesis
Assume that the theorem holds for all $I$ with $\card I = n$.

This is the induction hypothesis.

Induction Step
Now, suppose that $\card I = n + 1$.

Let $j \in I$.

Let $J = I \setminus \set j$.

By Cardinality Less One, $\card J = n$.

By the induction hypothesis, there exists a mapping:
 * $\ds g: J \to \bigcup_{i \mathop \in J} S_i$

such that:
 * $\forall i \in J: \map g i \in S_i$

By the definition of a non-empty set, there exists a $y \in S_j$.

Hence, there exists a mapping:
 * $\ds f: I \to \bigcup_{i \mathop \in I} S_i$

such that:
 * $\ds \forall i \in I: \map f i = \begin{cases}

\map g i & : i \ne j \\ y & : i = j \end{cases}$

Then:
 * $\forall i \in I: \map f i \in S_i$

as desired.