For many millenia, man's knowledge of the external world was limited to what he could detect with his senses. Living in a purely natural environment, he found his place therein, like the other animal species.
Man's senses, however, are not always better than those of animals: certain insects can see ultraviolet light; the bat can hear ultrasonic vibrations, dogs have a more highly developed sense of smell. So it is not because he has a sensory system superior to those of animals which has enabled man to conquer a privileged place for himself in nature. It is his ability to reflect upon the plain evidence of his senses, to analyse it, to compare various pieces of information with one another, and to interpret the result with his mind.
Among our senses, those which enable us to appreciate the arrangement in space of all the objects of the world around us, give us more valuable information than do the others. Thanks to this idea of arrangement in space we can attribute larger or smaller dimensions to objects.
Many animals of course possess the idea of space and have some knowledge of the sizes and distances away of various other animals, plants and objects in their environment. But this knowledge is confused and cannot be raised to the status of a science because the animals have no idea of measuring lengths or distances by a voluntary experimental act.
There are many species of animals which can count, but there is not a single one which has the faculty to measure. Thus even in the field of medium distances (such as the size of his own body, distances between himself and other objects) man has, since remotest antiquity, had a great superiority over animals in his ability to measure.
But it is only in the past few hundred years, with the invention of the microscope and telescope and other instruments of measurement, that man has made his most prodigious bound forward. Today, the electron microscope permits us to see details one hundred times smaller than ever before. In bacteriology it has provided valuable information about the structure of viruses and bacteria; in metallography it has made possible extensive study of metallic surfaces and crystals. The 200-inch Mt. Palomar telescope, on thé other hand, has extended man's vision of the universe up to 720 million light yearsthe distance of the furthest galactic cluster yet observed.
During this pursuit of fresh knowledge, into the regions of both the extremely small and extremely large, man has not only encountered unknown and marvellous phenomena which have enriched his conception of the universe, but has also been able to transform his own existence by entirely new means developed in the process.