The second of the above opinions ascribes to the natural sciences alone laws in the sense of universally valid rules for phenomena, and therefore limits psychology in principle to the description of facts, which appear in their combinations to be arranged purely by chance or at will. This opinion rests obviously on a mistaken use of the conception of law. We are only allowed to consider those regularities in phenomena as according to law, which always repeat themselves in exactly the same manner. But there are in reality no such laws, not even in the natural sciences. For this principle is valid here: laws determine the course of phenomena only in so far as they are not annulled by other laws. Now because of the complex nature of all phenomena in general each process stands under the influence of many laws, and so it happens that just the most universal natural laws can never in experience be demonstrated in their full power. There is no law of dynamics which has a more universal validity than the so-called "law of inertia" or Newton's first law of motion. It can be formulated as follows: "A body in motion, and not acted on by any external force, will continue to move indefinitely in a straight line and with uniform velocity." It is obvious that this law can never and nowhere be realised in experience, since a case of independence from other external forces, which alter the motion, never and nowhere exists. And yet the law of inertia is for us an infallible law of nature, since all real processes of motion may be looked upon as lawful modifications of that ideal case (never existing in concrete experience) of a motion not acted upon by any external influences.