HERMANN VON HELMHOLTZ 



sources of knowledge, the internal and external, we 

 obtain ideas, which may be simple or complex, and 

 ideas always deal with modes, substances or relations. 

 It is clear, therefore, that our knowledge, accord- 

 ing to these doctrines, must be mainly gained from 

 experience. 



Spinoza, Descartes and Leibnitz, on the other 

 hand, held that the mind, by its own powers, could 

 transcend the limits of experience and reach the truth. 

 The pantheistic monism of Spinoza implied unity of 

 substance, this substance having the fundamental 

 qualities of thought and extension ; God, ourselves, and 

 the world were one. A mode of extension (an ex- 

 ternal object), and the idea of an object are, in the 

 language of Spinoza, the same thing expressed in 

 two different ways. To understand a sensory im- 

 pression we must have an idea of the affected as 

 well as of the affecting body. 



Descartes conceived all external bodies to be ex- 

 tended substances, while the soul is a thinking sub- 

 stance without extension. External bodies are real, 

 because we are conscious of the dependence of sensa- 

 tions on external causes. Soul and body interact, 

 touching at one point, the pineal gland, and thus 

 body and spirit constitute a dualism, but the mode 

 of interaction is incomprehensible. Why did Des- 

 cartes attach such importance to an obscure little 

 organ, now known to be an abortive eye ? Leibnitz 

 introduced his strange system of monads, a monad 

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