Condillac developed his concept of empirical sensationism, and demonstrated "lucidity, brevity, moderation, and an earnest striving after logical method.

[7] His next book, the Traité des systèmes, is a vigorous criticism of those modern systems which are based upon abstract principles or upon unsound hypotheses.

His polemic, which is inspired throughout by Locke, is directed against the innate ideas of the Cartesians, Malebranche's faculty-psychology, Leibniz's monadism and pre-established harmony, and, above all, against the conception of substance set forth in the first part of the Ethics of Baruch Spinoza.

He questioned Locke's doctrine that the senses give us intuitive knowledge of objects, that the eye, for example, naturally judges shapes, sizes, positions, and distances.

The fourth section deals with the desires, activities and ideas of an isolated man who enjoys possession of all the senses; and ends with observations on a "wild boy" who was found living among bears in the forests of Lithuania.

He rejects the medieval apparatus of the syllogism; but is precluded by his standpoint from understanding the active, spiritual character of thought; nor had he that interest in natural science and appreciation of inductive reasoning which form the chief merit of J. S. Mill.

Some might claim that Condillac's anti-spiritual psychology, with its explanation of personality as an aggregate of sensations, leads straight to atheism and determinism.

He always asserts the substantive reality of the soul; and in the opening words of his Essai, "Whether we rise to heaven, or descend to the abyss, we never get outside ourselves—it is always our own thoughts that we perceive," we have the subjectivist principle that forms the starting-point of Berkeley.

Condillac promoted an expressionist theory of linguistic creation that anticipates the prime features of later thoughts about language by German theorist Johann Gottfried Herder (1744–1803).

Progress is marked by a rational development and use of resources; decline is precipitated by bad behavior from the upper classes that then trickles down to the workers, encouraging excess, luxury, and false prices that harm the masses.

Condillac saw the remedy to this as "vrai prix," a true price created by the unimpeded interaction of supply and demand, to be achieved by complete deregulation.

By advocating of a free market economy in contrast to the prevailing contemporary policy of state control in France, Condillac influenced classical liberal economics.

James Mill, who stood more by the study of concrete realities, put Condillac into the hands of his youthful son with the warning that here was an example of what to avoid in the method of psychology.

A modern historian has compared[11] Condillac with Scottish Enlightenment philosopher and pre-evolutionary thinker Lord Monboddo, who had a similar fascination with abstraction and ideas.

In France Condillac's doctrine, so congenial to the tone of 18th century philosophism, reigned in the schools for over fifty years, challenged only by a few who, like Maine de Biran, saw that it gave no sufficient account of volitional experience.

Early in the 19th century, the romantic awakening of Germany had spread to France, and sensationism was displaced by the eclectic spiritualism of Victor Cousin.

Biographical details and criticism of the Traité des systèmes in J. P. Damiron's Mémoires pour servir a l'histoire de to philosophie au dixhuitieme siècle, tome iii.