Why do mathematical Platonists believe numbers exist independently of human minds?

Mathematical Platonists believe that numbers and other mathematical objects exist independently of human minds because they view these entities as abstract objects—things that are nonphysical, nonmental, and nonspatiotemporal.1

Their belief is grounded in several key arguments:

• Objectivity of Mathematical Truth: Platonists argue that mathematical statements are literally true and describe a reality that exists regardless of human thought. They contend that if numbers were merely mental ideas, mathematical truths would become contingent on human existence; for example, if all humans ceased to exist, the statement "2 + 2 = 4" would remain true, suggesting it refers to an objective, independent reality.12
• Rejection of Alternatives: Platonists argue that competing views like psychologism (which treats numbers as mental ideas) and physicalism (which treats them as physical piles) fail to account for the nature of mathematical discourse, such as the existence of infinite sets or the methodology of mathematical proof, which is not empirical.2
• Indispensability: Many Platonists rely on the indispensability argument, which posits that because mathematical theorems are essential to the success of empirical scientific theories, those theorems must be true, implying the existence of the abstract objects they describe.2