By means of sense-experience we learn many things about the physical world-we perceive countless physical things, processes,
and events,
as well as the interaction of our own bodies with these things in nature.
But if our knowledge ended there,
we would have no means of dealing effectively with the world.
The kind of knowledge we acquire through the sciences begins only when we notice regularities in the course of events. Many events and processes in nature occur the same way over and over again.
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.