Notes on Hume's Treatise
by G. J. Mattey
PART III. OF KNOWLEDGE AND PROBABILITY
§ X. Of the influence of belief
The reasoning of the above section can be turned on the account of probable reasoning: we are unaccustomed to such a system.
The chief spring of human action is a perception of pain and pleasure, either as impression or idea. "Impressions always actuate the soul, and that in the highest degree; but 'tis not every idea which has the same effect." But both influence the will, which prevents a harmful one-sidedness (unprincipled action vs. restless action).
Raising an idea to the vivacity of an impression gives it a similar degree of influence on the passions. This is an additional argument for the system.
The passions influence belief: a coward easily believes there is danger, "as a person of a sorrowful and melancholy disposition is very credulous of every thing, that nourishes his passion." The emotion enlivens the object. Quacks incite belief through the passions of amaze and surprise.
The fancy may be satisfied though falling short of belief.
Truth and reality influence the fancy when they are imprinted by the habit associated with causality. "Belief must please the imagination by means of the force and vivacity which attends it; since every idea, which has the force and vivacity, is found to be agreeable to that faculty."
When the imagination is over-stimulated, there is no way to distinguish between the true and the false, "but every loose fiction or idea, having the same influence as the impressions of the memory, or the conclusions of the judgment, is receiv'd on the same footing, and operates with equal force on the passions." (cf. Appendix, 630-2) Poetry can have an enlivening effect.