Notes on Hume's Treatise
by G. J. Mattey
PART III. OF KNOWLEDGE AND PROBABILITY
§ V. Of the impressions of sense and memory
In our causal reckonings, there are three elements to be explained:
1) The original impression
2) The transition to the idea of the connected cause and effect
3) The nature and qualities of those ideas.
The ultimate cause of sense impressions is "perfectly inexplicable by human reason." The Cartesian trichotomy, object, ourselves, God, is invoked. But in the present case, all we need is coherence of perceptions to draw our inferences, "whether they be true or false; whether they represent nature justly, or be mere illusions of the senses."
The distinction of imagination from memory cannot be found in the simple ideas, which perfectly copy the original impressions. Nor can the arrangement of complex ideas be the source of the distinction. Although memory preserves order and imagination is free, this quality cannot be used to distinguish them, given the comparison problem. The only difference, then, can lie in the force and vivacity, in which memory is superior to imagination. But with time, the ideas of memory degenerate and we lose the use of our criterion. Conversely, the idea of the imagination may gain force and vivacity, as in the case of liars, who come to believe their own lies.
Recapitulating: our causal reasoning begins with sense and memory, but their only distinguishing marks are vivacity. "To believe is in this case to feel an immediate impression of the senses, or a repetition of that impression in the memory." Thus the causal judgment begins with a forceful and lively perception, which "lays the foundation of that reasoing, which we build upon it, when we trace the relation of cause and effect."