Six strengths, measured — and the states a graha can be found in
Every other document in this set asks what a graha signifies. This one asks whether it is in any condition to deliver. Ṣaḍbala is the classical attempt to make that quantitative: six independent sources of strength, each scored, summed into a total, and compared against a published minimum for that graha. It is the closest thing in the tradition to a measurement.
Treat the number as a summary rather than a verdict. A graha can clear its minimum on temporal and natural strength while sitting in a duḥsthāna owning the 8th, and the arithmetic will not tell you that. What ṣaḍbala does well is settle comparisons — which of two candidates for a yoga is actually the stronger, which of two malefics will dominate — and flag a graha whose apparent dignity is not supported underneath.
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Natural strength, and the minimum required
Naisargika bala is fixed for all charts — the grahas rank by brightness, in sevenths of a rūpa. The required minimum is what a graha's total ṣaḍbala must reach to be counted strong; note that Budha needs the most and is given the second-least naturally, which is why a strong Mercury is less common than it appears.