Final answer:
Gaunilo's island is different from Anselm's definition of God because the island is contingent, relying on other conditions for its existence, whereas Anselm's God is necessary, existing in and of itself without dependence on anything else.
Step-by-step explanation:
Why is Gaunilo's island different from Anselm's definition of God? The answer is B) Gaunilo's island is contingent, while God is necessary. Anselm's ontological argument, as it relates to the existence of God, distinguishes between necessary and contingent being. A necessary being, such as God in Anselm's framework, means that God's existence does not depend on anything else and is not contingent upon any conditions or context. By contrast, Gaunilo's critique using the concept of a perfect island is pointing out that even a supremely perfect island would remain a contingent entity, depending on various conditions for its existence and not inherently necessary. Anselm argues that by the very concept of God as 'a being than which nothing greater can be conceived,' God's existence is not contingent but logically necessary both in the understanding and in reality.
Furthermore, while Gaunilo's island is conceived as a physical entity with limits, God is posed as a metaphysical absolute, not bound by physicality or finitude. Gaunilo's challenge serves to question Anselm's leap from conception to existence, emblematic later in the critiques of thinkers like Immanuel Kant, who argued against existence being treated as a predicate. Following this line of thought, the idea of the perfect island (or any perfect item) does not inherently possess existence in reality simply because we conceptualize it as such.
Finally, Anselm's argument is integrally linked to the idea of God being the necessary being, the cause of itself, and the basis by which anything exists at all, as argued by Aquinas as well. Whereas Gaunilo's perfect island lacks these divine characteristics of necessity, eternality, and foundational existence.