Pythagoras and his followers, who maintained that abstract numbers exist outside the mind.

All rational people refuted this idea, and so did the philosophers who came after them. They said:

“Abstract number and abstract magnitude exist only in the intellect, not in external reality. What exists outside the mind are the things that are numbered and the things that are measured: discrete bodies that can be counted, such as the stars, and continuous bodies that can be measured, such as the celestial spheres. These alone are characterized by continuous and discrete quantity in the external world.”

The disciples of Plato, for their part, affirmed that intellectual universals—what they call the Platonic Forms—exist outside the mind and claimed that they are primordial and eternal, and they even posited an additional dimension.