The Quadrivium:—
[4] Arithmetic, [5] Geometry, [6] Music, and [7] Astronomy

[4] ARITHMETIC —
Number in itself, which is a pure abstraction; that is, outside of space and time.

The Quadrivium with Gene Odening, Gnostic Media Podcast episode #050 (start here)

Quadrivium – Number, Geometry, Music, & Cosmology with John Martineau, Gnostic Media Podcast episode #103 (intermediate)

The Golden Ratio with Dr. Scott Oslen, Gnostic Meida podcast episode #108 (advanced)