🙂 ) Here is D.T. Suzuki’s interesting (and rather elaborate!) explanation of their representing, in his interpretation, the basic forms and emptiness which are the source of all:
“The circle-triangle-square is [Rinzai Master 仙厓義梵 Sengai Gibon (1750-1837)’s] picture of the universe. The circle represents the infinite, and the infinite is at the basis of all beings. But the infinite in itself is formless. We humans endowed with senses and intellect demand tangible forms. Hence a triangle. The triangle is the beginning of all forms. Out of it first comes the square. A square is the triangle doubled. This doubling process goes on infinitely and we have the multitudinosity of things, which the Chinese philosopher calls ‘the ten thousand things’, that is, the universe. The trouble with us linguistically-minded beings is that we take language realistically and forget that language is of no significance whatsoever without time. In truth, language is time and time is language. We thus come to think that there is in the beginning of the world a something which is real and concrete, such as a world of galaxies which though formless and nebulous is yet real and tangible. This is the foundation of the universe on which we now have all kinds of things, infinitely formed and varied. It is thus that time itself begins to be thought of as something concrete and real. A circle turns into a triangle, and then into a square, and finally into infinitely varied and varying figures. In the same way the Biblical account of creation has turned into historical truth in the minds of many. But Zen is very much against such fabrications. There is another and a more traditional interpretation that may be given to these three figures of forms. Sengai was familiar with Shingon, the mantra sect of Buddhism, as well as Zen. He liked Shingon because it taught the identity of the bodily existence (rupakaya) with ultimate reality (dharmakaya). The bodily existence is here represented by a triangle which symbolizes the human body in its triple aspect, physical, oral (or intellection), and mental (or spiritual). The quadrangle represents the objective world which is composed of the four great elements (mahabhuta), earth, water, fire and air. The Dharmakaya, the ultimate reality, is the circle here, that is, the formless form. We generally hold a dichotomous view of existence, form (rupakaya) and formless (arupa), object and subject, matter and spirit, and think they contradict each other and are mutually exclusive. Both Shingon and Zen, however, oppose this view and hold that what is form is formless or empty (sunya), that is, they are identical.
~D.T.Suzuki
From Jundo’s commentary –
http://www.greenshinto.com/wp/2016/01/31/zen-and-shinto-4-square-triangle-circle/
Reality is Dharma 