In geometry we have access to perfect squares and circles, but no such objects exist in the material world. The objects that we think about in mathematics must be real, since they are most certain. Since they could not exist in the material world, there must be another realm in which they exist that is even more real, the realm of forms. Unless there are forms it seems that some form of relativism is true. Relativism is the view that all that exist are subjective points of view about such things as truth, beauty, and justice.
Relativism is self-defeating.
We do often objectively discuss and argue about concepts like beauty and truth and justice, and by this dialectic process we come to a better understanding of them. If there are forms of beauty, truth, and justice then it is possible to objectively criticize subjective points of view about these things. We already believe that the more objective a value is, the more real it is. This is shown by the way we distinguish real values from apparent ones. Only detachment from all particular desires for objects and persons can be completely objective.