Answer: Just like there is no one except the Creator and the created being, there is also nothing except a circle and a line. A triangle, a square, a hexagon (the unification of two triangles, an upper and lower one), and a diagonal line are artificial, temporary forms in the process of correction.
Only lines and circles exist in nature, and they turn into one whole in the World of Infinity. Where the intention is only for the sake of bestowal, the line becomes equal to a circle. The round Light of the World of Infinity spreads out in the form of a circle, passes through a line, and achieves a round shape again. Then, it comes down again through a line and comes to a circle again, and so on.
All the circles are connected to each other with lines, and the line segments interlace with circles. There are lots of such garlands. Atik, Arich Anpin, and YESHSUT consist of a multitude of such wonderful systems. Sometimes, space is depicted this way.
Each of us consists of such structures, and there isn’t a single detail that is not built from circles and lines. This is caused by the fact that GAR (the upper three Sefirot) of each degree consists of circles, and ZAT (the seven lower Sefirot) consists only of lines. Thus, both are present in each Partzuf and its part.
In other words, you have a connection with the World of Infinity through the circle, and through the line, you have connection to the line. Thus, you consist of these two shapes.
From the Daily Kabbalah Lesson 11/19/10, Talmud Eser Sefirot