All events of nature occur in space and in time. Whatever we perceive, whatever record we receive through our senses, always is attached to, and contained in, space and time. But are space and time real existing things? Have they an absolute reality outside of our mind, as a part or framework of nature, as entities--that is, things that are? Or are they merely a conception of the human mind, a form given by the character of our mind to the events of nature--that is, to the hypothetical cause of our sense perceptions? Kant, the greatest and most critical of all philosophers, in his _Critique of Pure Reason (Kritik der Reinen Vernunft)_, concludes that space and time have no absolute existence, but are categories--that is, forms in which the human mind conceives his relation to nature. The same idea is expressed by the poet-philosopher Goethe in his dramatic autobiography _Faust_ (in the second part), when he refers to the "Mütter," to the marriage of Achilles and Helena "outside of all time." It is found in ancient time. So Revelation speaks of "there should be time no longer" (hoti chronos ouketi estai). The work of the great mathematicians of the nineteenth century--Gauss, Riemann, Lobatschefsky, Bolyai--offered further evidence that space is not an empirical deduction from nature, but a conception of the mind, by showing that various forms of space can be conceived, differing from one another and from the form in which the mind has cast the events of nature (the "Euclidean" space). Finally, physical science, in the theory of relativity, has deduced the same conclusions: space and time do not exist in nature by themselves, as empty space and empty time, but their existence is only due to things and events as they occur in nature. They are relative in the relation between us and the events of nature, so much so that they are not fixed and invariable in their properties, but depend upon the observer and the conditions of observation. We can get an idea of how utterly our perception of nature depends on the particular form of our time conception by picturing to ourselves how nature would look if our time perception were 100,000 times faster, or 100,000 times slower.