He asserted, that this gland is so suspended in the midst of the brain, that it could be moved by the slightest motion of the animal spirits: further, that this gland is suspended in the midst of the brain in as many different manners, as the animal spirits can impinge thereon; and, again, that as many different marks are impressed on the said gland, as there are different external objects which impel the animal spirits towards it; whence it follows, that if the will of the soul suspends the gland in a position, wherein it has already been suspended once before by the animal spirits driven in one way or another, the gland in its turn reacts on the said spirits, driving and determining them to the condition wherein they were, when repulsed before by a similar position of the gland.

He further asserted, that every act of mental volition is united in nature to a certain given motion of the gland.

For instance, whenever anyone desires to look at a remote object, the act of volition causes the pupil of the eye to dilate, whereas, if the person in question had only thought of the dilatation of the pupil, the mere wish to dilate it would not have brought about the result, inasmuch as the motion of the gland, which serves to impel the animal spirits towards the optic nerve in a way which would dilate or contract the pupil, is not associated in nature with the wish to dilate or contract the pupil, but with the wish to look at remote or very near objects.

Lastly, he maintained that, although every motion of the aforesaid gland seems to have been united by nature to one particular thought out of the whole number of our thoughts from the very beginning of our life, yet it can nevertheless become through habituation associated with other thoughts; this he endeavours to prove in the Passions de l'ame, I.50.

He thence concludes, that there is no soul so weak, that it cannot, under proper direction, acquire absolute power over its passions.