Descartes says that to have a mind is to be able "to affirm, to deny, to conceive, to know, to hope, to expect, to will," and so forth. He denies that animals have this capacity. I personally have cats, and believe that they have the capacity for some of those tasks. They "seem" to enforce their will upon me more than i can to them, despite my greatest efforts, and when it comes to conditioning them for any task you really have to wonder who is teaching who. BF Skinner on the other hand says that if we don't need psychological categories for animals, or plants, we don't need them for humans. The reason I believe he said this, is that in Skinner's view there is no such thing as free will, all of our choices are dictated by past experiences with reinforcement or punishment. So what seems to be the "mind" is actually just a rough computation of what would provide the best form of reinforcement. This view doesn't cover things like desire, no matter how much I know I shouldn't do something, if the desire is great enough to do it, I probably will, with no disregard to negative reinforcement or possible punishment.