A Power of the Soul or of the Object?
In the Cartesian corpus, the expression 'force de Vame is in fact very rare. It fleetingly appears in the Treatise ofMan, and possesses only two occurrences in The Passions of the Soul, the first one in article 36,15 the second one in the title of article 49.16 In the specific context of this last treatise, which deals with how the soul can manage its passions, this notion includes a moral dimension which it had not in the first one. This dimension is to be taken into account, but in some cases it just comes in addition to the purely objective and functional one, according to which the French word ‘force corresponds to the Latin vis and not to fortitude.17 The soul exerts this vis whenever an intention or determination of the will induces or seems to induce in the body a specific movement or physical process. This happens in voluntary motion and in the soul’s struggle against its present passion, but also in the voluntary mode of imagination and recollection. In this last case, the physical process remains purely internal to the brain; but in all those cases, the relevant phenomenon is a certain movement of the pineal gland in which the soul has its ‘principal seat’.
Our major problem will of course concern the movement of the gland, which seems to have been the sole cause of the scandal. What has disturbed many readers is the power that Descartes seems to attribute to the soul to make the little gland lean to one definite side, in order to engage a definite motion or to start a definite process ending with a definite result, either in the body, or out of it, or even in the soul itself (with its own perceptions). In the Passions of the Soul, art. 47, Descartes actually says:
As the little gland in the middle of the brain is capable of being driven from one side by the soul and from the other by the animal spirits, which... are only bodies, it often happens that these two impulses are in opposition and the stronger one prevents the other from taking effect. 
One can therefore speak of a sort of struggle ‘between the impetus by which the spirits impel the gland to cause in the soul the desire for some thing, and that by which the soul repels [the gland] by the volition it has to shun that very thing’.19
This is very surprising language: we are driven here to attribute directly to the soul, to the soul in itself, a certain effort which is supposed to be quantifiable (being stronger or weaker than the physical effort of the spirits on the gland). However, this account must be counterbalanced or at least mitigated with the details given in article 44. Descartes says here: ‘It is not always the volition to excite some movement or some other effect in us which enables us to excite it; this varies according to nature or habituation that has diversely joined each movement of the gland to each thought’.20 We are taught here that the soul never directly applies a certain force or effort to the little gland. The force applied to the gland rather belongs to the object of its thought.
The same principle reappears in article 47, in a passage which seems crucial to me. What exactly happens when the soul intends to fight against its present passion?
The will not having the power to excite the passions directly, as has already been said, it is constrained to employ artifice [d'user d'industrie] and apply itself to attend successively to different things. If the first of these happens to have the strength to change the course of the spirits for a moment, it may happen that the following one does not have it and that they immediately revert, because the previous disposition in the nerves, heart and blood is unchanged—which makes the soul feel driven almost at the same time to desire and not to desire the same thing.21
The soul’s action on the course of the animal spirits does not just imply the shaping of a certain volition, but also the appeal to the imagination which is the sole thing able to act on the spirits in a direct mode. And thus, it is the appearance of an impression or image in the brain, and not a movement simply considered as such, which will be here the deciding factor. There is no evidence of any exception to this condition.