Many scholars believe that Aristotle's works as we have them today are little more than lecture notes. Many of his works are extremely compressed and thus baffling to beginners. Nowhere is this more evident than in the Metaphysics — Ibn Sina (Avicenna), one of the greatest Medieval Islamic philosophers, said that he had read the Metaphysics of Aristotle forty times, but still did not understand it. Only later, after having read al-Farabi's, Purposes of the Metaphysics of Aristotle, did he understand Aristotle's book. 
In the 19th century, with the rise of textual criticism, the Metaphysics was examined anew. Critics, noting the wide variety of topics and the seemingly illogical order of the books, concluded that it was actually a collection of shorter works thrown together haphazardly. Werner Jaeger further maintained that the different books were taken from different periods of Aristotle's life. Everyman's Library, for their 1000th volume, published the Metaphysics in a rearranged order that was intended to make the work easier for readers.