i 4 2 HERMANN VON HELMHOLTZ 



Since we never can perceive the objects of the external 

 world directly, but only from their action upon our nervous 

 mechanism, the question obviously presents itself, how in 

 the first instance we ever got into touch with the real world 

 by means of our nervous sensations? We must postulate 

 the presence of external objects as the cause of our nervous 

 excitation, since there can be no effect without a cause : but 

 this dictum can be no law of experience, we already need 

 it for the knowledge that there are any objects at all in the 

 space around us. Yet it cannot come from the internal ex- 

 perience of our self-consciousness, since we regard the self- 

 conscious acts of our will as free. Hence we must fall back 

 on Kant's conclusion that all our thoughts and acts, the greatest 

 as the least, are founded on our confidence in the unalterable 

 uniformity of nature, and that the axiom, 'no effect without 

 a cause/ is a law of our thought prior to all experience. Among 

 the papers left by Helmholtz is the following interesting note 

 on this subject: 



'The Law of Causation (the presupposed uniformity of 

 nature) is a mere hypothesis, and not otherwise demonstrable. 

 No previous uniformity can give proof of future uniformity. 

 The sole test of any hypothesis is, try if it be so, and you 

 will find out (best by experiment, where possible). In com- 

 parison with other hypotheses which enunciate special laws 

 of nature, the Law of Causality is exceptional in the following 

 ways: (i) all others presuppose it; (2) it gives us our sole 

 possibility of knowing something we have not observed ; (3) it 

 is the necessary foundation of premeditated action ; (4) we 

 are reduced to it by the natural mechanics of our combinations 

 of ideas. Hence we are induced by the strongest motives to 

 desire its validity. It is the groundwork of all our thoughts 

 and acts. Until we have it we cannot test it; therefore we 

 can but believe in it, act upon it, and find it justified by fair 

 tests. We must anticipate the consequences; then the con- 

 sequences will be its confirmation. We must be aware that 

 we anticipated the result ; then we shall be aware of the law. 

 Thinking means seeking for uniformity ; judging, that we have 

 found it. Hence, without the law of causation there can be 

 no thought. No thought without acceptance of the law of 

 causation is tautology ; query, are we justified in thinking, and 



