The first factor in the cognitive process is an impression on a sense-organ, which, occasioned by external motion, continues onward to the heart and from this center gives rise to a reaction. The perception or sensation which thus arises is entirely subjective, a function of the knower merely, and in no way a copy of the external movement. The properties light, color, and sound, which we believe to be without us, are merely internal phenomena dependent on outer and inner motions, but with no resemblance to them. Memory consists in the lingering effects or residuary traces of perception; it is a sense or consciousness of having felt before _(sentire se sensisse meminisse est_), and ideas are distinguished from sensations as the perfect from the present tense. Experience is the totality of perceptions retained in memory, together with a certain foresight of the future after the analogy of the past. These stages of cognition, which can yield prudence but not necessary and universal knowledge, are present in animals as well as men. The human capacity for science is dependent on the faculty of speech; words are conventional signs to facilitate the retention and communication of ideas. As the memory-images denoted by words are weaker, fainter, and less clearly discriminated than the original sensations, it comes to pass that a number of similar ideas of memory receive a common name. Thus abstract general ideas and generic concepts arise, to which nothing real corresponds, for in reality particulars alone exist. The universal is a human artefact. The combination of words into propositions, being an addition or subtraction of arbitrary symbols or marks, is called judgment; the combination of propositions into syllogisms, inference; the united body of true or demonstrated principles, science--hence mathematics is the type of all knowledge. In short, thought is nothing but calculation and the words with which we operate are mere counters; he who takes counters for coin is a fool. Animals lack reason, _i.e._, this power of combining artificial symbols.