3 Faith then, as it has been elsewhere defined, is “the firm support of those who hope, the evidence of things which are not seen.” If they are not seen, how are they evidenced to be? What! Whence are these things which you see, but from That which you see not? To be sure you see somewhat that you may believe somewhat, and from that you see, may believe what you see not. Be not ungrateful to Him who has made you see, whereby you may be able to believe what as yet you can not see.
God has given you eyes in the body, reason in the heart; arouse the reason of the heart, wake up the interior inhabitant of your interior eyes, let it take to its windows, examine the creature of God. For there is one within who sees by the eyes. For when your thoughts within you are on any other subject, and the inhabitant within is turned away, the things which are before your eyes you see not. For to no purpose are the windows open, when he who looks through them is away. It is not then the eyes that see, but some one sees by the eyes; awake him, arouse him.
For this has not been denied you; God has made you a rational animal, set you over the cattle, formed you after His Own image. Ought you to use them as the cattle do; only to see what to add to your belly, not to your soul? Stir up, I say, the eye of reason, use your eyes as a man should, consider the heaven and earth, the ornaments of the heaven, the fruitfulness of the earth, the flight of the birds, the swimming of the fish, the virtue of the seeds, the order of the seasons; consider the works, and seek for the Author; take a view of what you see, and seek Him whom you see not.
Believe on Him whom you see not, because of these things which you see. And lest you think that it is with my own words that I have exhorted you; hear the Apostle saying, “For the invisible things of God from the creation of the world are clearly seen by those things which are made.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)