The second Risalat of the Ikhwan us Safa is on geometry (handasat). Geometry is of two kinds: aqliyya (mental) and hissiya (physical). Physical shapes that can be seen fall in the latter category, while shapes and objects that are not seen but merely imagined fall in the first category.
The line, the plane and the object are the three basic geometric units. The object has three dimensions known as the tul (height), araz (width) and umuq (depth). The dot is the basic unit of a line, several dots drawn together make a line. The line is the basic unit of a surface, several lines put together make a plane. The plane is the basic unit of an object, many planes form an object.
Geometry is one of the many sciences that are necessary in our life. A single person, given his short life span, cannot learn all the sciences; however several individuals with different sciences and skills working together can create a highly developed society. Geometry provides a standard system of measurement that aids in transactions between people of different trades.