Iron rusts,
but gold does not.
Chickens lay eggs,
but dogs do not.
Lightning is followed by thunder.
Cats catch mice,
but cows don't.
(Even to speak of a cat or a cow is to have noted some regularity-that some characteristics regularly recur, or go together.)
Amidst the constant diversity in our daily experience of nature,
we try to find regularities:
we trace the thin red vein of order through the flux of experience.