Why did Leopold Kronecker oppose Georg Cantor's ideas about infinite numbers?

Leopold Kronecker’s opposition to Georg Cantor’s work on infinite numbers was rooted in a fundamental philosophical disagreement regarding the nature of mathematical existence.

Kronecker was a proponent of a constructive approach to mathematics, famously asserting that "God made the integers, and all the rest is the work of man." He believed that only the integers truly existed and insisted that all mathematical concepts should be reducible to these positive whole numbers.12

In contrast, Cantor’s set theory and his introduction of transfinite numbers relied on nonconstructive existence proofs—arguments that demonstrate an object must exist without providing a specific method to construct it. Kronecker viewed Cantor’s ideas as overly metaphysical and rejected the notion that infinite sets could have an "actual" existence. This deep-seated ideological conflict led Kronecker to vehemently oppose Cantor’s research, even going so far as to block his academic advancement at the University of Berlin.123