From Reaching for the Invisible God, p. 149:
Mention of the Holy Spirit summons up up confusion. If a person or group claims, “The Bible says,” you can look for yourself. If they claim, “The Spirit told me,” where can you look? There lies the problem: by definition the Spirit is invisible. Jesus drew a parallel for Nicodemus: “The wind blows wherever it pleases. You hear its sound, but you cannot tell where it comes from or where it is going.” How can we detect a presence that has no shape, no identifiable form? Nevertheless, no one who wants to know God can ignore the Spirit.
The Christian who is enamored with rationalism will always squirm at the mention of the spirit. Who can systematically dissect the whispers of him deep inside the hearts of men? That’s impossible! Better just stick with the books. Except the books speak of the spirit.