Some creatures are moving because they are agitated, unsatisfied, or driven by craving, and this in the Buddhist context invokes the sense of frailty or weakness. Similarly when one is firmly grounded, tranquil and at rest, this expresses a condition of greater strength and stability.
Andy Olendzki commenting on the metta sutta
How different our take on "only the strong survive" when we look at strength and weakness from this context. It is strength of character and mind that survive. The truth is our shelter, not dominating power born from fear, agitation, or craving. Brute strength may get some temporary results, but nothing lasting.
Craving and fear are normal, human qualities of mind, but when acted upon mindlessly indicate frailty or weakness, an inability of the mind to be with itself and respond skillfully. Strength of mind gives us the power to be with things as they are, to respond skillfully and appropriately given current circumstances and long term effects.
