from association, viz. those mentioned in the third, fourth, and sixth corollaries, so the sensations and ideas have the same three connexions. Hence the whole doctrine of association may be comprised in the following theorem, viz.
If any sensation A, idea B, or muscular motion C, be associated for a sufficient number of times with any other sensation D, idea E, or muscular motion F, it will, at last, excite d, the simple idea belonging to the sensation D, the very idea E, or the very muscular motion F.
The reader will observe, that association cannot excite the real sensation D, because the impression of the sensible object is necessary for this purpose. However, in certain morbid cases, the idea is magnified so as to equal, or even overpower, sensible impressions.
In order to verify this proposition, it is necessary to inquire, what connexions each automatic motion has gained by association with other motions, with ideas, or with foreign sensations, according to the third, fourth, and sixth corollaries of the last proposition, so as to depend upon them, i.e. so as to be excited no longer, in the automatic manner described in the nineteenth proposition, but merely by the previous introduction of the associated motion, idea, or sensation. If it follow that idea, or state of mind (i.e. set of compound vibratiuncles), which we term the will, directly, and without our perceiving the intervention of any other idea, or of any sensation or motion, it may be called voluntary, in the highest sense of this word. If the intervention of other ideas, or of sensations and motions (all which we are to suppose to follow the will directly), be necessary, it is imperfectly voluntary; yet still it will be called voluntary, in the language of mankind, if it follow certainly and readily upon the intervention of a single sensation, idea, or motion, excited by the power of the will: but if more than one of these be required, or if the motion do not follow with certainty and facility, it is to be esteemed less and less voluntary, semi-voluntary, or scarce voluntary at all, agreeably to the circumstances. Now, if it be found, upon a careful and impartial inquiry, that the motions which occur every day in common life, and which follow the idea called the will, immediately or mediately, perfectly or imperfectly, do this, in proportion to the number and degree of strength in the associations, this will be sufficient authority for ascribing all which we call voluntary in actions to association, agreeably to the purport of this proposition. And this, I think, may be verified from facts, as far as it is reasonable to expect, in a subject of inquiry so novel and intricate.
