Matthew Zsebik wrote:
What inspired O'Sensei to use those shapes to describe things in Aikido?
Those or anything else.
Their use appears in several other cultures like ancient Greece where the circle triangle and square appear in Plato's Timaeus. Do Asian religions and philosophy often use these?
Those and anything else.
Basically the discourse-style is very promiscuous. Any one thing can become anything else. This is why science uses the term "mysticism" as a term of derision. I happen to think this misses the point, but a lot of people take the map for the territory and turn the mystic heuristics into things.
Kannon, Goddess of Compassion becomes the ferocious God Fudoo. Shinto becomes Buddhism. A mountain in Japan becomes the mountain in India where Buddha preached becomes a mountain in Buddhist heaven.
Circle Triangle Square are "elemental" shapes and quite suggestive of association--no-brainers in the mystic pantheon of mnemonics and meditative devices. (Fred, step in here if I over-step myself.)