The image of the Pythagorean tetrad stems from an early ninth-century manuscript in the Royal Library of Belgium which contains, amongst other texts, Isidore of Seville’s De natura rerum [On the Nature of Things] The figure is an illustration of Chapter XI of this book: ‘on the constituents of the world’.
The Pythagorean tetrad is all about the number four. tetra comes from the Greek τετράς: number of four. In this representation it encompasses the four classical elements with their attributes. The elements are at the cross’s extremities:
- fire (ignis) is on top; it is dry (sicca) and hot (calidus);
- on the right we have air (aer), hot (calidus) and wet (humidus, here wrongly spelled as numidus);
- beneath is aqua (water); it is wet (humidus) and cold (frigida);
- on the left we find earth (terra), cold (frigida) and dry (sicca).
All divine creations are made of a combination of these four elements:
- the world (mundus);
- the annual cycle (annus) with its four seasons: summer (aestas), spring (ver), winter (hiems) and autumn (autumnus);
- man and his four humours: yellow bile (cholera), blood (sanguis), phlegm (humour), and black bile (melancholia).
It provides an excellent summary of all the topics which will be presented and discussed during the Symposium: the earth element stands for land cartography; the air for celestial cartography; the water for the sea charts and the fire for maps related to warfare and fortifications plans.