Having named the _a priori_ forms of the understanding, how does Kant show that by their means our knowledge of nature has validity? Because when the understanding functions, it prescribes these forms to perception. Impressions would remain vague and formless, if we did not think them; by means of thought we weld impressions into objects and give them a coherent reality. This is exactly what is meant by understanding. If nature were an independent thing and prescribed laws to the understanding, the laws would never be universal and necessary. The universality of the laws of nature can be explained only by supposing that the understanding prescribes its laws to nature, not to nature as a Thing-in-Itself, but only so far as it appears in sense-perception. Universal and necessary knowledge of nature is possible only if the connections and relations of nature are absolutely identical with the modes of thought. The categories of the understanding have objective validity, therefore, because the laws of the understanding are the fundamental laws of nature. The understanding has given such laws to nature. _A priori_ and therefore universal and necessary, synthetic and therefore creative, the world consists of objects under laws of the understanding. There are as many kinds of natural objects as there are categories of the understanding. If we will examine what we call the world of nature, we shall find that many of its objects have never been perceived. Man has only partly explored the earth, and there are vast regions in space that he has never seen. He has never seen the South Pole, and the North Pole only recently; he has never seen the other side of the moon, and there are myriads of stars beyond even the reach of his telescope. These are not perceptible things, and yet they are the objects of the understanding――objects of knowledge. How is it possible? It would not be possible if the laws of nature were limited to the empirically perceived facts. It is possible because the laws of the understanding are the laws of nature and apply everywhere, whether the thing is actually perceived or not. The moon must have another side because the human understanding conceives all substances in this way; the law of cause and effect obtains beyond the stars, and at the South Pole, even though they have never been perceived. The world of physical objects, or in other words the world of objects of the understanding, consists of both possible and actually perceived objects. If the laws of nature were prescribed by nature to the mind, then the world of objects would consist only of actually perceived objects.