Ainulindalë (Quenya for Music of the Ainur) is the first chapter of The Silmarillion, a collection of fictional histories by J. R. R. Tolkien, edited and published posthumously by his son, Christopher Tolkien. As part of the Tolkien Middle-earth cycle, Ainulindalë plays the role of its cosmogony, or 'creation story'. Many of the themes and storylines revealed in the later histories (The Lord of the Rings, for example) find first expression in Ainulindalë.
While it is true that the Ainur are Ilúvatar’s thoughts embodied, they each have a life of their own, and are expected to utilize their ‘freedom’ by cultivating the grand theme. Only in the future, at the ‘end of days’, will all the created beings of Ilúvatar fully understand not only the divinely provided concepts and themes they each personally embody, but how each relates to all the others and fits (as per Ilúvatar’s intentions) in the entire greater scheme.
The Ainur see in the Vision that there are things none of them remember composing, and things they perhaps remember composing but didn’t at the time fully understand. The Children of Ilúvatar are first mentioned here, the future home of the Children (Arda – ‘the Realm’, i.e. the Earth ) is spotted, and some positively fascinating advice is given: don’t read too much into the relative size of the Earth as compared to the entire Universe, or be overly impressed with the immensity of Space compared to, say, the delicacy and complexity of design in a mustard seed. Many of the Ainur, including Melkor, become enamored of the Earth, though Melkor still wants to dominate it and the Children.