This discovery of the law of gravity and its consequences is the most impressive achievement that the logical power of the human mind has yet been capable of. I do not want to say that there have not been men with a capability for abstract thought equal or greater to Newton and the other astronomers – some of whom prepared the way for his discovery, some of whom exploited it – but never before had a subject as fitting as the confounding and intricate movement of the planets been comprehended under a general law, a subject which previously fed only the astrological superstitions of uneducated observers.
Based on this greatest of examples, a series of other branches of physics emerged, namely optics and the theory of electricity and magnetics. In the search for general laws of nature, those sciences which are based on experiments have an advantage over those based on observation, because they can manipulate the conditions at will and therefore need only a small number of characteristic cases in order to find the rule. The validity of the law must, admittedly, be tested on more complex cases. Thus, the physical sciences advanced relatively rapidly once they found the right methods. They have made it possible not only to cast our gaze into the very beginnings of time, when nebulae balled themselves up into stars and began to glow through the power of their compression, not only to investigate the component parts of the sun's atmosphere – the chemistry of the most distant fixed stars is certainly not far off – but they have also taught us to make use of the powers in our immediate natural environment, to put them in the service of our will.
From everything I have discussed here, it should be clear how different the intellectual activity in these sciences is from those I first spoke of. A mathematician needs no memory for individual facts, a physicist needs very little. Intuitions based upon the recollection of similar cases can be useful, in order to put things on the right initial track, but they only have value when they lead to a strictly defined and precisely limited law. With nature, there is no doubt that we are dealing with a causal nexus which knows no exceptions. We must therefore continue to work until we have discovered laws which are valid in every case. Before this is accomplished, we cannot be satisfied. Only in this form does our knowledge achieve victory over space and time and the forces of nature.