§ 13. The difference between similar things which are not congruent cannot be made intelligible by understanding and thinking about any concept. They can only be made intelligible by being intuited or perceived. For example, the difference of chirality is of this nature. So, also, is the difference seen in mirror images. Right hands and ears are similar to left hands and ears. They are not, however, congruent. These objects are not things as they are apart from their appearance. They are known only through sensuous intuition. The form of external sensible intuition is space. Time is the form of internal sense. Time and space are mere forms of our sense intuition and are not qualities of things in themselves apart from our sensuous intuition.