It may be pretended, that the resistance which we meet with in bodies, obliging us frequently to exert our force, and call up all our power, this gives us the idea of roce nd power. It is this nisus, or strong endeavour, of which we are conscious, that is the original impression from which this idea is copied. But, first, we attribute power to a vast number of objects, where we never can suppose this resistance or exertion of force to take place, to the Supreme Being, who never meets with any resistance; to the mind in its command over its ideas and limbs, in common thinking and motion, where the effect follows immediately upon th will, without any exertion or summoning up of force, to inanimate matter, which is not capable of this sentiment. Secondly, This sentiment of an endeavour to overcome resistance has no known connexion with any event: What follows it we know by experience; but could not know it a priori. It must, however, be confessed that the animal nisus, which we experience though it can afford no accurate precise idea of power, enters very much into that vulgar, inaccurate idea, which is formed of it.
This is from Hume, An Enquiry Concerning Human Understanding, Chapter VII.
It seems to me that Hume has underestimated the real challenge provided to his theory by the sentiment of nisus or endeavor. Some of my thoughts on why:
1) Can we actually make any sense of our feeling this overcoming-of-resistance without thinking of it in terms of exercise of power (both of endeavor and of resistance to it)? To be sure, we can't, a priori, determine whether this endeavor will have an effect - but it seems that any sentiment of endeavor is very plausibly characterized as the sentiment of one's own exercise of power resisted by something else's exercise of power. Hume always thinks of 'power' or 'agency' as something that has an effect; but isn't this a bit odd? Isn't it a part of our idea of power or agency that usually it can be exercised but fail (if certain conditions are met).
2) It is true that we attribute power to things to which our sentiment of endeavor can't be attributed. But (a) this doesn't prevent the sentiment of endeavor from really being a sentiment of (one kind of) power; (b) the reason we don't attribute to endeavor to God is that there is no adequate resisting power - but our sentiment of endeavor seems to be an impression of exercising-power-against-a-resisting-exercise-of-power. This resistance can be greater or less; we can take endeavor as an idea of the exercise of power, and let the power of resistance approach to zero, and we have an effortless exercise of power. Hume might consider this effortless endeavor to be a fiction or even a straightforward error; but in the Treatise he does similar sorts of things (e.g., with regard to geometry or to the coherence of our perceptions), so it's hard to say why it would be completely ruled out; (c) we don't attribute our sentiment of endeavor to inanimate matter, but we don't attribute our sentiment of extension to inanimate matter, either. We still can say that inanimate matter is extended; the only reasons that could be proposed for denying parallel treatment to endeavor are that endeavor isn't something really sensed in the sensation of endeavor, or that it is essentially conscious in nature. These would need to be argued.
3. Hume needs to say _why_ it enters into the vulgar idea of power, if it has nothing to do with power. Why would such a confusion be possible?
I suspect Hume could present a coherent response to the endeavor theory; but his dismissing it in a footnote doesn't really do justice to it.