47. The fundamental principle of Condillac's theory is, that sensation is the sole operation of the mind; and that whatever exists in our mind is nothing more than the sensation transformed in various ways. Prior to sensible impressions, this philosopher admits no faculty; the development of sensation is all that fecundates the soul, not by exciting its faculties, but by generating them. The school of the Aristotelians took, indeed, sensations for the starting-point, but did not consider them as producing intelligence; on the contrary, they were very careful to mark the limits of the sensitive faculties, and of the understanding in which they recognized a peculiar and innate activity altogether superior to the faculties of the sensible order. We have only to open any one of the innumerable works of this school, to meet on every page such words as _intellectual force_, _light of reason_, _participation in the divine light_, and others in the same style, in which a primary activity of our mind, not communicated by sensations, but prior to them all, is expressly recognized. The acting intellect, _intellectus agens_, which figures so much in this ideological system, was a standing condemnation of the system of transformed sensation advocated by Condillac. 48. The Aristotelians, governed by their favorite idea of explaining every thing by matter and form, modified the meaning of these words according to the exigencies of the objects to which they applied them, and considered the faculties of the soul as a class of forces incapable of acting unless united to a form which brought them into action. Thus they explained sensations by species, or forms, which placed the sensitive power in act. The imagination was a force which, although it sometimes rose above the external senses, contained nothing but species of the sensible order, subject also to the necessary conditions of this faculty. These species were the forms which placed the imaginative force in act, and without which it could not exercise its functions. The Aristotelians, after having thus explained the phenomena of the external senses, and of the imagination, undertook to explain those of the intellectual order; and in this they displayed their genius by inventing an auxiliary which they named the _acting intellect_. The necessity of making two principles in seeming contradiction accord, was the reason of this invention. On the one hand the Aristotelians held that our cognitions all flowed from the senses; and on the other they asserted that there was an essential and intrinsic difference between feeling and understanding. Having drawn this dividing line, the sensitive and intellectual orders were separated; but as it was on the other side requisite to establish some communication between these two orders, it was necessary for them, if they wished to save the principle, that all our ideas come from the senses, to discover some point where the two channels might unite.