There is another consideration. Whatever is continuous disappears from our perception, whereas anything that is momentarily tangible becomes visible to us. This is shown when we start on a sea voyage. At first the noise of the engines is almost unbearable, but as we go on we get accustomed to it, so that after four or five days we find that we do not notice the noise any more, while at the same time we can hear the least whisper of a friend speaking to us. The continuous noise is now no longer audible unless we stop, to pay attention to it.