As long as there is any phenomenon left out of the systematic coherence that you have discovered, science is discontented and insists that either the system is wrongly or imperfectly conceived or else the facts have not been correctly stated.
This demand for "a coherent and comprehensive statement of the whole field of fact" comes solely from reason.

How do we get it?
We have no ground in experience for insisting that the world shall be regarded as intelligent, as "all hanging together and making up one system." But reason insists upon it.

This gives us "a kinship between the mind of man and the universe he lives in." Now, when man puts his great question to the universe, and to every phenomenon in that universe, _Why ?_--Why is this what it is, what my reason recognises it to be?
is he not in truth asking, What is this thing's purpose?
What is it doing in the universe?
What is its part in the coherent system of all-things-together?
Now there is in our experience already one principle which does answer the question "Why ?" in such a way as to raise no further questions; that is, the principle of Purpose.

Let us take a very simple illustration.

Across many of the hills in Cumberland the way from one village to another is marked by white stones placed at short intervals.