Calvin on common grace (or all truth comes from God)

Whenever we come upon these matters in secular writers, let that admirable light of truth shining in them teach us that the mind of man, though fallen and perverted from its wholeness, is nevertheless clothed and ornamented with God's excellent gifts. If we regard the Spirit of God as the sole fountain of truth, we shall neither reject the truth itself, nor despise it wherever it shall appear, unless we wish to dishonor the Spirit of God. For by holding the gifts of the Spirit in such slight esteem, we condemn and reproach the Spirit himself. What then? Shall we deny that the truth shone upon the ancient jurists who established civic order and discipline with such great equity? Shall we say that the philosophers were blind in their fine observation and artful description of nature? Shall we say that those men were devoid of understanding who conceived the art of disputation and taught us to speak reasonably? Shall we say that they are insane who developed medicine, devoting their labor to our benefit? What shall we say of all the mathematical sciences? Shall we consider them the ravings of madmen? No, we cannot read the writings of the ancients on these subjects without great admiration....Those men whom Scripture calls "natural men" were, indeed, sharp and penetrating in their investigation of inferior things. Let us, accordingly, learn by their example how many gifts the Lord left to human nature even after it was despoiled of its true good.

John Calvin, Institutes 2.2.15 (as cited by Michael Horton, "A Shattered Vase: The Tragedy of Sin in Calvin's Thought" in A Theological Guide to Calvin's Institutes, ed. by David W. Hall and Peter A. Lillback)