"The weakness of men. Yes, the Astartes united Terra and conquered the galaxy. Behind their guns and swords we forge accross the stars and claim many thousands of worlds for the Emperor. You created the Imperium, of that I'm sure. But without us weak, frail men what would you be? Who pilots the ships that carry you? Who grows the crops that feed you, makes the weapon you wield and raises the children that would be your future generations? Not the Astartes."
Excerpt from Raven's Flight, Conversation between Prifactor Valarius and Chapter Master Bran