Why did Immanuel Kant reject St. Anselm of Canterbury’s ontological argument?

Immanuel Kant rejected the ontological argument formulated by St. Anselm of Canterbury primarily because he challenged the underlying assumption that existence is a predicate or attribute.123

Anselm’s argument posits that God is the greatest conceivable being and that existence is a perfection; therefore, a being that exists in reality is greater than one that exists only in the mind, necessitating that God must exist. Kant, most notably in his Critique of Pure Reason, argued that existence is not a property that can be added to a concept to make it more perfect.41235

According to Kant, stating that something exists does not add a new attribute to the concept of that thing; rather, it asserts that the concept is instantiated in reality. Because existence is not a predicate, one cannot define a being into existence by including it as a necessary quality of perfection. Consequently, Kant maintained that whether a concept is instantiated is a factual question that cannot be determined a priori through logical definition alone.132