The more I read Hume, the more shallow and careless he appears to me.
Thus, “an idea assented to feels different from a fictitious idea, that the fancy alone presents to us: And this different feeling I endeavor to explain by calling it” drum roll… “a superior force, or vivacity, or solidity, or firmness, or steadiness.” (AToHN, 1.3.7)
But surely, “greater” force, etc. that allegedly distinguishes a true idea from a false one, or an idea believed from one doubted, is a difference in degree; whereas truth is distinct from falsity, and assent from doubt in kind. How long shall the Humean submarginal changes in degree accumulate before they become a marginal change in kind? Hume does not tell us. If idea A is slightly more forceful or vivid to me than another idea B, am I permitted or required to conclude that A is true, and B is false? Implausible.
It seems to me rather that (1) people very often believe in fictions with remarkable fanaticism, such that the false ideas are extremely forceful and vivid in their minds; and (2) doubt is marked by a feeling of uneasiness, agitation, and discomfort from the professed ignorance that drives one to search for truth. Upon discovering an apparent truth or upon finalizing a decision, there is a feeling of peace and pleasure and accomplishment: now I know; now I am ready to execute the plan of action that has just crystallized.
Moreover, an idea that is assented to, i.e., understood to be true, is similar to a figment of the imagination in that the falsity of the latter is similarly assented to. Why should propositions known to be false or phantasms known not to correspond without a doubt be any less vivid, etc., given that of neither doubt is entertained?
Is Hume saying perhaps that ideas assented to are more important (for action, say) than mere fancies, and so by some instinct, more effort is put into their presentation to the intellect? By virtue of this, the former are felt more forcefully. Hmm… this is hardly self-evident. I’m going out on a limb here; this is reading into Hume far more than he deserves.
Force and vivacity seem to play little role in the phenomenology of these things.