INTRODUCTION 287 



certainties ; such knowledge can, I believe, only be attained by the 

 following method. 



Do not let us be imposed upon by dogmatic utterances which are 

 nearly always ventured with little thought ; let us carefully collect 

 such facts as we can observe, let us make experiments wherever we can, 

 and when experiment is impossible let us marshal all the inferences that 

 we can draw from analogy, and let us nowhere make a dogmatic 

 pronouncement : by this method we shall be able gradually to attain 

 a knowledge of the causes of many natural phenomena, including 

 perhaps even those that now appear to us the most incomprehensible. 



Since, then, the limits of our knowledge as to what occurs in nature 

 neither are nor can be fixed, I shall endeavour, by the use of such facts 

 as have been collected, to determine in this third part what are the 

 physical causes which confer on certain animals the faculty of feeling, 

 of producing for themselves the movements which constitute their 

 actions, and, lastly, of forming ideas and of comparing these ideas, so as 

 to obtain judgments : in short, of performing various intelligent acts. 



The principles which I shall set forth on this matter will as a rule 

 be such as to fill us with an inward moral conviction, although it is 

 impossible to prove positively their accuracy. It seems that, with 

 regard to many natural phenomena, this order of knowledge is alone 

 possible for us ; and yet its importance cannot be called in question 

 in innumerable cases where we are called upon to form judgments. 



If the physical and the moral have a common origin, if ideas, thought 

 and even imagination are only natural phenomena, and therefore really 

 dependent on organisation, then it must be chiefly the province of 

 the zoologist, who makes a special study of organic phenomena, to 

 investigate what ideas are, and how they are produced and preserved, 

 in short, how memory renews them, recalls them and makes them 

 perceptible once more ; from this it is only a short way to perceiving 

 what are thoughts themselves, for thoughts can only be invoked by 

 ideas ; lastly, by following the same method and building up from 

 original perceptions, it may be possible to discover how thoughts 

 give rise to reasoning, analysis, judgments and the will to act, and how 

 again numerous acts of thought and judgments may give birth to 

 imagination, a faculty so fertile in the creation of ideas that it even 

 seems to produce some which have no model in nature, although in 

 reaUty they must be derived from this source. 



If all the acts of the intellect, into the causes of which I am now 

 enquiring, are only phenomena of nature, that is to say, acts of the 

 organisation, may I not hope, by acquiring a thorough knowledge 

 of the only means by which the organs perform their functions, to 

 discover how the intellect may give rise to the formation of ideas and 



