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They are his own reflections as far as I can tell. IMO the suttas don't suggest that the 4 brahma vihΔras keep each other in check. My interpretation is that each of the 4bv on its own, like every other major pre Buddhist concept that the Buddha borrowed and then redefined, like the 5 faculties, 'dukkha', 'arahant', becomes subservient and must conform to the Buddha's teaching on 4 noble truths, and right view especially. So the upekkha as a brahma-vihΔra, essentially becomes the same upekkha of 4th jhΔna. Whereas upekkha pre-Buddhist, would probably mean one has the equanimous observation of someone with the understanding of karma, rebirth, but obviously not the full understanding of a Buddha and an Arahant that ends rebirth.