Avicenna on abstraction

The theory of abstraction is one of the most puzzling parts of Avicenna's philosophy. What Avicenna says in many passages about the human intellect's capacity to derive universal knowledge from sense-data seems to plainly contradict passages in the same works about the emanation of knowledge from the active intellect, a separately existing substance. When he maintains that "considering the particulars [stored in imagination] disposes the soul for something abstracted to flow upon it from the active intellect", 1 he appears to combine two incompatible concepts in one doctrine: either the intelligible forms emanate from above or they are abstracted from the data collected by the senses, but not both.