 Despite being well known, Leibniz’s concept of apperception is not necessarily well understood. In particular, the place of apperception in the three-fold classifications given at the end of the previous section—of three kinds of perceptions and of simple substances—is not agreed upon, despite the fact that this would seem to be of considerable importance. (See Kulstad 1991a for discussion.) A common understanding is that for Leibniz apperception is distinctive of spirits and is not present in even the highest of animals beneath humans. While there is evidence that Leibniz at least sometimes adopts this position, there is also evidence that he sometimes endorses the view that (at least some) beasts also apperceive. Since we may assume that at a minimum apperception involves consciousness (though not necessarily certain higher forms of consciousness, e.g., self-consciousness, or reflective consciousness, in one sense or another), this leads to some uncertainty as to whether Leibniz assigns consciousness to beasts, that is, whether he does or does not agree with the famous Cartesian principle that beasts are not conscious but only material automata.