The source on which Joyce calls most particularly is Plato's Timaeus but in fact the Yeatsian context of the most obvious allusions to it suggests that Joyce actually came to the Timaeus through A Vision. Whatever his direct source may have been, Joyce treated the Platonic theme as he did Vico and Blavatsky and used the theories for all he was worth. The description in the Timaeus of the creation of the World-Soul, if not the archetype of this particular mystic symbol in western literature, is at least one of its most important early appearances': 

'Next he cleft the structure. . - lengthwise into two halves, and  laying the two so as to meet in the centre in the shape of the letter X, he bent them into a circle and joined them, causing them to meet themselves and each other at a point opposite to that of their original contact: and he comprehended them in the motion that revolves uniformly on the same axis, and one of the circles he made exterior and one interior. The exterior motion he named the motion of the Same, the interior that of the Other. And the circle of the Same he made revolve to the right by way of the side, that of the Other to the left by way of the diagonal'. (36 B. C.) 

It is clear that Finnegans Wake is woven out of two such strands of World-Soul, represented by the Shem-Shaun polarity. There are two extremes to the function of this polarity, between which the line of development swings to and fro: when their orbits are in close proximity they war with each other and - at a moment of exact equilibrium - even manage to amalgamate, while at the other extreme there is total incomprehension and a failure to communicate, symbolised by the point of farthest separation of the orbits.

The two structural meeting-points are at the coincident beginning and end, 1.1 and IV, and at the centre, II.3 - that is, diametrically opposed on the sphere of development. The strands spread out from the initial point of contact - the conversation of Mutt and Jeff, who have just met - widen throughout Book I and converge until they meet once more during the Butt and Taff episode, at the end of which they momentarily fuse, only to cross over and separate again during Book III before the final meeting (identical with the first) when Muta and Juva converse. 'Mutt and Jeff' and ' Muta and Juva' are the same event looked at from opposite sides; the book begins and ends at one of the two nodal points, while, when Joyce has cut the circles and stretched them out flat, the other nodal point falls exactly in the centre of the fabric. Represented in this way, the basic structure of Finnegans Wake thus looks rather like a figure 8 on its side, which forms the ' zeroic couplet' oo, or the symbol for 'infinity'.            

Chapter II.3 was the last part of Finnegans Wake to be written; This, the longest chapter in Finnegans Wake, is the most important of all, the nodal point of the major themes, a clearing-house and focus of motifs. Only Book IV and the opening pages, preparing the way for a new cycle, can compare with II.3 in this respect.

Hart, Clive / Structure and motif in Finnegans wake