6 parts of the path required:
According to Madara, from chapter 462, page 12:
from the younger:
1. Sage's Body
2. His Life Force
3. His Physical Energy
and from the elder:
4. Sage's Eyes
5. His Powerful Chakras
6. His Spiritual Energy
A.
i. Madara stated he 'acquired' A Senju part (1. or 2. or 3.) by means of his battle with Hashirama.
ii. In chapter 510, page 11, Madara states that Izanagi is the combination of one part vitality, the Physical Energy (3. above) which forms the basis of "yang" power...
B.
i. Madara Is an Uchiha and has An Uchiha part by virtue of the elder being the forerunner of all Uchiha (either 5. or 6.).
ii. In chapter 510, page 11, Madara states that Izanagi is the administration of imagination, the Spiritual Energy (6. above) which forms the basis of "yin" power....
C. And has now re-acquired the Eyes, the rinnegan (4. above), the 3rd in total of six paths of the Sage (parts). Chapter 510, page 17: "You are the third of the sixth paths."
As in the third part of my being complete of 6 total needed.
My theory is Madara is Madara only in his Spiritual Energy (6.) An energy of Spirit which drives his being and identifies itself as Uchiha Madara. His Body, I believe, is a shell. One which encapsulates an exact copy of Senju Hashirama's Physical Energy(3.) 'captured' by Zetsu upon touching him during the battle a VoTE.
Thus, as in chapter 510, page 12, Madara states that: "Two of the six paths are now one being!"
And now, the Rinnegan, the 3rd part of 6, which "were mine to begin with" chapter 511, page 17.
He lacks only these 3 "Steps towards becoming complete":
1. Sage's Body
2. His Life Force
5. His Powerful Chakras
And targets Naruto to fulfill one of these three (1. or 2.)