INTRODUCTION 289 



us from seeing them or even keeping them in a vessel for making 

 experiments with, only manifest their existence by their effects ? 

 These eifects constitute a cogent proof that no other cause could have 

 produced them. It is, moreover, easily ascertained that the visible 

 fluids, which penetrate the medullary substance of the brain and nerves, 

 are only nutritive and adapted for secretion ; but that they are too 

 slow in their movements to give rise to the phenomena either of 

 muscular movement, feeling, or thought. 



In the light of these principles, which restrain the imagination within 

 its proper limits, I shall first show how nature originally succeeded in 

 creating the organ of feeling, and by its means the force productive 

 of actions : I shall afterwards proceed to consider how (by means of a 

 special organ for intellect), ideas, thoughts, judgments, memory, etc., 

 may have arisen in the animals which possess such an organ. 



