  The hierarchy of monads mentioned above has a corollary in Leibniz's epistemology. Monads are more or less perfect depending upon the clarity of their perceptions, and a monad is dominant over another when the one contains reasons for what happens in the other. But some monads can also rise to the level of souls when, for example, they experience sensations, that is, when their perceptions are very distinct and accompanied by memory. This is a position occupied by animals. Furthermore, some souls are sometimes also in a position to engage in apperception, that is, to reflect on their inner states or perceptions. As Leibniz tells us in the Principles of Nature and Grace, “it is good to distinguish between perception, which is the internal state of the monad representing external things, and apperception, which is consciousness, or the reflective knowledge of this internal state, something not given to all souls, nor at all times to a given soul” (G VI 600/AG 208). The point that Leibniz wants to make is clearly an anti-Cartesian one: it is not the case that animals lack souls and are mere machines. There is a continuum here from God, angels, and human beings through animals to stones and the dull monads which underlie the muck and grime of the world; and this continuum is not solely to be understood in terms of the comparative clarity of the mind's perceptions but also in terms of the kinds of mental activity possible for a particular being. Indeed, according to Leibniz, animals operate not as mere automata as they do in the Cartesian philosophy, but rather have fairly sophisticated mental faculties. Even a dog, for example, is capable, by virtue of its memory, of having a perception of a prior perception: “[t]hat is why a dog runs away from the stick with which he was beaten, because his memory represents to him the pain which the stick caused him” (G VI 600/AG 208). While this resembles reasoning, it is not the kind of reasoning that human beings are capable of; for the mental processes of the dog are “only founded in the memory of facts or effects, and not at all in the knowledge of causes” (ibid.). At the same time, Leibniz is quick to add that the mental activity of the dog is the same as the mental activity of human beings in three fourths of their actions, for most of us most of the time are not actually reasoning from causes to effects. And yet we are different from the beasts, Leibniz believes. Some creatures are capable of knowing the necessary and eternal truths of logic and mathematics and a priori truths (from cause to effect), and they “are properly called rational animals, and their souls are called minds.” (G VI 601/AG 209) As Leibniz says, “These souls are capable of performing reflective acts, and capable of considering what is called ”I“, substance, soul, mind – in brief, immaterial things and immaterial truths. And that is what makes us capable of the sciences of demonstrative knowledge” (ibid.). Thus, what makes human beings (and higher minds) special is the capacity, via apperception, to formulate a conception of the self. Indeed, as we see in this passage, Leibniz suggests that rationality itself follows from the capacity for reflection: we begin with a conception of the self; we move from this point to thinking of being, of substance, of God; and we become aware as well of eternal and necessary truths. Rationality, however, is really only the ability to form “indubitable connection[s] of ideas” and to follow them to their “infallible consequences” (ibid.). In other words, animals and most human beings most of the time are purely empiricists; a rational person, however, is one who can engage in genuine a priori reasoning, moving from knowledge of a true cause via deduction to necessary effects.