The Fellowship of the Ring
For the black horses can see, and the Riders can use men and other creatures as spies, as we found at Bree. They themselves do not see the world of light as we do, but our shapes cast shadows in their minds, which only the noon sun destroys; and in the dark they perceive many signs and forms that are hidden from us: then they are most to be feared. And at all times they smell the blood of living things, desiring and hating it. Senses, too, there are other than sight or smell. We can feel their presence – It troubled our hearts, as soon as we came here, and before we saw them; they feel ours more keenly. (1.11.116)
"As the Power grows, its proved friends will also grow, and the Wise, such as you and I, may with patience come at last to direct its courses, to control it. We can bide our time, we can keep our thoughts in our hearts, deploring maybe evils done by the way, but approving the high and ultimate purpose: Knowledge, Rule, Order; all the things that we have so far striven in vain to accomplish, hindered rather than helped by our weak or idle friends. There need not be, there would not be, any real change in our designs, only in our means. (2.2.156)
"And now we must enter the Golden Wood, you say. But of that perilous land we have heard in Gondor, and it is said that few come out who once go in; and of that few none have escaped unscathed."
"Say not unscathed, but if you say unchanged, then maybe you will speak the truth," said Aragorn. "But lore wanes in Gondor, Boromir, if in the city of those who once were wise they now speak evil of Lothlórien [...] Perilous indeed [...] fair and perilous; but only evil need fear it, or those who bring some evil with them." (2.6.53-54)