Final answer:
The discussion on existence in logic versus reality involves distinguishing between concepts in the mind (intra-mental) and actual existence (extra-mental). Immanuel Kant challenged Anselm's ontological argument about God's existence by stating that conceptual existence doesn't equate to real existence. Anselm's argument from possibility to necessity and Descartes' perfection-based reasoning as a priori proofs are contrasted with the empirical evidence requirement of a posteriori arguments.
Step-by-step explanation:
Existence in Logic vs. Existence in Reality
The philosophical debate over the nature of existence as it pertains to objects or entities such as God, the perfect island, or a unicorn involves distinguishing between mental or intra-mental concepts and actual extra-mental existence. Immanuel Kant's view is that existence is not a predicate, meaning that just because we can conceive of the most perfect being, the greatest conceivable being (GCB), it doesn't follow that this being actually exists outside of our minds.
Anselm's ontological argument suggests that the very concept of God as 'that than which nothing greater can be conceived' implies God's necessity to exist both in the mind and in reality. This is because a God that exists in reality is greater than one that exists only in thought. However, Kant counters this by asserting that compulsory conceptual thinking of an entity's existence does not constitute its actual existence. The presupposition of such existence requires independent empirical evidence beyond mere ideas.
In examining the nature of existence further, we venture into the realm of cosmology and ontology. Anselm and Descartes provide a priori arguments based on reason, distinct from a posteriori cosmological arguments which derive from experiences within the world. The reality of the mind is challenged, with various views including behaviorism, and the hard problem of consciousness exploring the depth of our self-awareness.