Soul and body duality and Plato

1. The idea of a perfect circle is a human concept.
2. The idea of a perfect circle doesn't derive from sense experience.
3. All ideas gained in our earthly existence derive from sense experience.
So
4. Some human concepts aren't ideas gained in our earthly existence.

Plato.

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'2' is suspicious because the concept of a perfect circle could be culturally taught.
'3' is a problem because we might have some ideas not gained from sense experience. For example, the idea of the square root of minus 1 in math. This is an imaginary and very useful concept that does not correspond to any sense experience.