A natural body is a substance that is extended in three dimensions. According to Avicenna, “[N]atural body is a substance in which one can posit an extension and another extension crossing it perpendicularly and a third extension crossing both of them perpendicularly; its having this characteristic is the form whereby it becomes a [natural] body” (Cure “Physics” I.2.1 M-R 156). This shows that for Avicenna, any substance that can posit the three dimensions: length, depth, and breadth can be considered a natural body. Moreover, natural bodies must be autonomous. A natural body must have both that which explains its being susceptible to or potentially having three dimensions and that which explains it’s actually having three dimensions.
Similar to Aristotle, Avicenna asserts that there are three main parts of the soul: the vegetative soul, the animal soul, and the human soul. Avicenna claims that each part has sub-faculties of their own. The vegetative soul is responsible for the faculty for taking in nutrition, which is for growth and reproduction. The animal soul is responsible for the motor faculty, which itself has two further divisions: (1) the faculty to incite motion and the faculty that produces motion, and (2) the perceptual faculty which is composed of the five inner senses and five outer senses. The five inner senses are: the ‘common’ sense, the faculty of retentive imagery, and cognitive faculty, estimative, and memory. The five outer senses are: touch, smell, taste, hear, and see. Lastly, Avicenna asserts that the human soul is divided into the practical intellect and the theoretical intellect.
In reference to the human soul, Avicenna asserts there are three levels of human understanding with corresponding potentialities. Avicenna asserts that all human beings are born with an innate ability of acquired understanding. The degree of potentiality that corresponds with this level of understanding is material potentiality. This baseline level of understanding is apparent in an infant’s ability for learning to read and write. This is the first level of understanding. The second level of understanding that humans possess is the acquired understanding. The degree of potentiality that corresponds with this level of understanding is possible potentiality. This level of understanding is apparent in a young child that knows the alphabet and can also write a few simple words. Furthermore, the third level of understanding is known as dispositional understanding. The degree of potentiality that corresponds with this level of understanding is perfect potentiality. This level of understanding is apparent in a fully literate person who can read and write whenever he or she pleases. Lastly, the final level of understanding is known as actual understanding. The degree of potentiality that corresponds with this level of understanding is absolute actuality. This level of understanding occurs in a person actively exercising his or her literacy whenever he or she pleases. This individual is 100% literate, even when they are not focused on the fact that they are not literate.