Well-Founded Induction

Theorem
Let $\struct {A, \prec}$ be a strictly well-founded relation.

Let $\prec^{-1} \sqbrk x$ denote the preimage of $x$ for each $x \in A$.

Let $B$ be a class such that $B \subseteq A$.

Suppose that:
 * $(1): \quad \forall x \in A: \paren {\prec^{-1} \sqbrk x \subseteq B \implies x \in B}$

Then:


 * $A = B$

That is, if a property passes from the preimage of $x$ to $x$, then this property is true for all $x \in A$.

Proof
$A \not \subseteq B$.

Then $A \setminus B \not = 0$.

By Strictly Well-Founded Relation is Well-Founded, $A \setminus B$ must have some strictly minimal element under $\prec$.

Then:
 * $\exists x \in \paren {A \setminus B}: \paren {A \setminus B} \cap \prec^{-1} \sqbrk x = \O$

so:
 * $A \cap \prec^{-1} \sqbrk x \subseteq B$

Since $\prec^{-1} \sqbrk x \subseteq A$:
 * $\prec^{-1} \sqbrk x \subseteq B$

Thus, by hypothesis $(1)$:
 * $x \in B$

But this contradicts the fact that:
 * $x \in \paren {A \setminus B}$

By Proof by Contradiction it follows that:
 * $\paren {A \setminus B} = \O$

and so:
 * $A \subseteq B$

Therefore:
 * $A = B$

Also see
Well-Ordered Induction, a weaker theorem that does not require the Axiom of Foundation.