In order to understand completely all the changes the different conditions produce in the imagination, it is absolutely necessary to be reminded that we imagine objects only by forming images of them, and that these images are nothing other than the traces the animal spirits make in the brain; that we imagine things more strongly in proportion as these traces are deeper and better engraved, and as the animal spirits have passed through them more often and more violently; and that when the spirits have passed through these traces many times, they enter there more easily than other places nearby, through which they have never passed, or have not passed as often.