After describing the nervous system and its functions, he discusses the nervous fluid.

His physiological views are based on those of Richerand's _Physiologie_, which he at times quotes.
Lamarck's thoughts on the nature of the nervous fluid (_Recherches sur le fluide nerveux_) are curious and illustrative of the gropings after the truth of his age.
He claims that the supposed nervous fluid has much analogy to the electric, that it is the _feu ethere_ "animalized by the circumstances under which it occurs." In his _Recherches sur l'organisation des corps vivans_ (1802) he states that, as the result of changes continually undergone by the principal fluids of an animal, there is continually set free in a state of _feu fixe_ a special fluid, which at the instant of its disengagement occurs in the expansive state of the caloric, then becomes gradually rarefied, and insensibly arrives at the state of an extremely subtile fluid which then passes along the smallest nervous ramifications in the substance of the nerve, which is a very good conductor for it.

On its side the brain sends back the subtile fluid in question along the nerves to the different organs.
In the same work (1802) Lamarck defines thought as a physical act taking place in the brain.

"This act of thinking gives rise to different displacements of the subtile nervous fluid and to different accumulations of this fluid in the parts of the brain where the ideas have been traced." There result from the flow of the fluid on the conserved impressions of ideas, special movements which portions of this fluid acquire with each impression, which give rise to compounds by their union producing new impressions on the delicate organ which receives them, and which constitute abstract ideas of all kinds, also the different acts of thought.
All the acts which constitute thought are the comparisons of ideas, both simple and complex, and the results of these comparisons are judgments.
He then discusses the influence of the nervous fluid on the muscles, and also its influence considered as the cause of feeling (_sentiment_).
Finally he concludes that _feu fixe_, caloric, the nervous fluid, and the electric fluid "are only one and the same substance occurring in different states." FOOTNOTES: [107] Charles Bonnet (1720-1793), a Swiss naturalist, is famous for his work on Aphides and their parthenogenetic generation, on the mode of reproduction in the Polyzoa, and on the respiration of insects.

After the age of thirty-four, when his eyesight became impaired, he began his premature speculations, which did not add to his reputation.