Aristotle's logical treatises, constituting what was later called the "Organon", contain the first systematic treatment of the laws of thought in relation to the acquisition of knowledge.In the first treatise, the "Categories", Aristotle gives a classification of all concepts, or notions, according to the classes into which the things represented by the concepts or notions, naturally fall. In the treatise styled "Analytica Priora", Aristotle treats the rules of syllogistic reasoning, and lays down the principle of induction. In the "Analytica Posteriora" he takes up the study of demonstration and of indemonstrable first principles. Besides, he treats of knowledge in general, its origin, process, and development up to the stage of scientific knowledge. From certain well-known passages in this treatise, and from his other writings, we are enabled to sketch his theory of knowledge. As was remarked above, Aristotle approaches the problems of philosophy in a scientific frame of mind. He makes experience to be the true source of all our knowledge, intellectual, as well as sensible. "There is nothing in the intellect that was not first in the senses" is a fundamental principle with him, as it was later on with the Schoolmen. All knowledge begins with sense-experience, which of course has for its object the concrete, particular, changeable phenomenon. But though intellectual knowledge begins with sense-experience, it does not end there, for it has for its object the abstract, universal, immutable essence. This theory of cognition is, so far, summed up in the principles: Intellectual knowledge is essentially dependent on sense-knowledge, and intellectual knowledge is, nevertheless, superior to sense-knowledge. How, then, does the mind pass from the lower knowledge to the higher? How can the knowledge of the sense-perceived (aistheton) lead to a knowledge of the intelligible (noeton)? Aristotle's answer is, that the mind discovers the intelligible in the sense-perceived. The mind does not, as Plato imagined, bring out of a previous existence the recollection of certain ideas, of which it is reminded at sight of the phenomenon. It brings to bear on the phenomenon a power peculiar to the mind, by virtue of which it renders intelligible essences which are imperceptible to the senses, because hidden under the non-essential qualities. The fact is, the individual substance (first substance) of our sense experience--this book, this table, this house--has certain individuating qualities (its particular size, shape, colour, etc.) which distinguish it from others of its species and which alone are perceived by the senses. But in the same substance, there is underlying the individuating qualities, its general nature (whereby it is a book, a table, a house); this is the second substance, the Essence, the Universal, the Intelligible. Now, the mind is endowed with the power of abstraction, generalization, or induction (Aristotle is not very clear as to the precise nature of this power) by which it removes, so to speak, the veil of particularizing qualities and thus brings out, or leaves revealed, the actually intelligible, or universal, element in things, which is the object of intellectual knowledge. In this theory intellectual knowledge is developed from sense-knowledge in so far as that Process may be called a development in which what was only Potentially intelligible is rendered actually intelligible by the operation of the active intellect. The Universal was in re before the human mind began to work, but it was there in a manner only potentially because, by reason of the individuating qualities which enveloped it, it was only potentially intelligible.